(No spoilers!) When I first heard of GoT I was excited. I had seen the intro, heard the mass buzz, and pegged it for a show where "something" battled "something else" and the theme was mad technological gimmicks. Mechanical machines, whether or not sentient or whatever. Well... no. I was quite disappointed once starting to watch the show that it was a medieval based drama. Worse yet, that fun mechanical intro? Well that's about as tech as it gets. I started reading the books after asking for them for my birthday. I kind of figured I'd go through them quite fast but did not anticipate about 3k of pages with very small letters :p No problem, just takes longer.
tl;dr: If you enjoyed the show, read the book. You can thank me later.
As for spoilers. I despise spoilers myself and promise I won't spoil anything beyond what you're bound to discover or things that are inherent to the show. If you're even more extreme on that, well, you probably already watched the show or read the books and I won't be able to spoil stuff for ya. Else, go watch. Now. That said...
As far as the show goes, I pulled myself through the first few episodes, kind of hoping for some kind of technological onslaught to follow the meager intro. While the tech stuff never really happens, the storyline caught me and I'm hooked now. What sells the show for me is that anyone may die at any point in the show for any reason. There may be lead roles, but any of them may die from a trivial act. Okay, most of them don't die in a whim but the point is that any action is exciting because you don't know whether or not some lead role will die when it may.
Compare that to a show like Dexter. It has uh, seven seasons? While season finales can be exciting in the "will he survive or naught" department, nearly any other episode does not. Spoiler? Hardly. It doesn't take an Einstein that the lead role with the name of the show won't be knifed midway the second season. Especially knowing in hind sight that there'll be five more after that. Not so in Game of Thrones. The first one had a small "wow" moment for me. The second one had a big one. There are multiple, don't worry, I'm talking about moments that had me stunned a bit personally.
So there are currently four tv show seasons that have aired. Each has 10 episodes if memory serves me correctly. There are five books and I think each season correlates to a book? So far that seems to hold for the first two books at least.
The problem with the tv show is that the story contains a massive trove of names. There are so many people names, even the vast number of family names, large and small, boggle the mind. In the second season I couldn't really tell anymore who was who and to which family it belonged, to whom it pledged fealty (loyalty) and what the current status was.
The book is much much better at this, though I still kind of recommend watching at least the first one or two tv seasons before reading the book. While the book holds little new, revisiting the story line is far from dull and the book reveals many connections and inside jokes that you probably missed in the tv show. The book is written in a first person perspective on various characters. Most importantly you get to know their true thoughts and intentions. In the tv show there's no narration so the only way you can figure out what somebody thinks is when they state it out loud. And even then you never know whether it's the truth or some ploy. So the book certainly has an edge there.
The names are also repeated in the book, over and over again, true names and nick names. In the show somebody may be introduced once, perhaps twice, but after that you're supposed to recognize them visually and remember yourself to which of the quadrillion story lines and family somebody belongs. I found it most confusing at first. It took me a while to get used to it and even in season three or four I was still struggling a bit at times. In season three somebody was referred to by nick name and I had no clue about whom they were talking until the real name was mentioned a bit later in that season. Even though it's obvious to me know that this name and nick should have been obvious to me from the start. Oh well.
The books clarity in this regard clarified so many things for me. There are family ties in the Deanerys story line that tie directly to The Wall storyline. Yeah, plural. I never even knew, or may have forgotten it if it was mentioned at some point. I never knew Jorah's backstory, in part let alone complete. Knowing it now makes certain scenes make so much more sense.
There's also a whole back story of the main story that feels left out in the tv show. No wonder since again, there's no narration so much of the back story can't be covered. The Targaryan family used to rule the throne. That much was clear. But to me not exactly how they came to power, how long they reigned, or how that ended. The old gods and the new, but I don't remember a good explanation of these gods. What's Kings Landing and why is that Wall so huge? Yeah that's right, it took the first book to clarify these things to me. It's not really a spoiler, just part of the back story. I think it mainly helps to get some gags and supports other back story lines. Like that of Ned's (dead) sister. I was completely oblivious to it. Don't recall ever learning about it in the tv show. Not even convinced these things are covered in the show at all, though I'll be sure to verify that later. Again, not really a spoiler, it's something that's briefly covered very early.
Reading the book opened my eyes to so many connections of the story line that it's almost like reading a new story, though key turning points and main story line arcs are still the same. In some cases I do view them in a new light though. Or see the deeper meaning, bigger importance of the whole thing.
For example, and this is not a spoiler until after you see the acts; Something that's a central point in season four apparently started by an event in season two. When watching season four I didn't even remember that at all. Reading the second book makes it clear that it ought to be weird that I didn't know, as the reasoning behind the event was quite influential to various parts of the story line. How could I not have known? In hindsight I do remember parts of the event being covered in the tv show, but certainly not as in depth as, apparently, it should have been covered.
Another example is one of the first beheading in story (this happens within like the first ten minutes, it's not really a spoiler, the story is super gory, better get used to it). While the act seems trivial, there are so many meanings to it that you discover later. The first book constantly refers to it in various points. You won't know what you were actually witnessing in that act until much much later, though you'll get more clues about it sooner than that. I felt the books clarity helps in "getting" these things better, since you know what was coming and understood what was happening better when reading it, reliving the story again.
I guess that's also a reason why I like reading the book in hindsight; I seem to appreciate the back story more knowing what's to come. Although it's mostly "vaguely knowing what's to come" since the show started four years ago. Like the "Jaqen" character was introduced much later than I remembered it, so for a while I was wondering whether this was something that the tv show did different. But at some point that seemed like an unlikely deviation and indeed it turned out I just remembered it wrong.
So for me, reading the book after watching the show (starting four years ago) still turns up some surprises, albeit not as surprising as they were at first. You're really just reading up to key story points and wondering "now? maybe now? oh that's because of ... so ... NOW? oh come on, NNOOW?". All whilst reading more back story overall and the connections between individual story lines.
The book, at least my box set edition, has two additions that can be a small help; a map and a small overview of characters per big (family) house. The map is much like what you see in the intro part of the tv show except you have plenty of time to review and it covers no castles in detail :) The second book does sport a map of Kings Landing in more detail.
The character overview can be helpful in case you forgot which knight was beheaded by which Lord _this_ time. The kings guard all look alike as do the Starks or the Freys. It's good to know that the overview in each book is NOT a spoiler of the book itself; the overview will reflect the state of affairs as it was at the beginning of the book. So if a character dies, is captured by somebody, or was captured and now released, the overview will not reflect this in the book where this occurs but only the next book. The second book gives a similar overview per story line and more details about the lesser houses.
As for storyline, well it may be hard to give a proper picture without giving away spoilers, but let's try anyways. The next part is only trivial main non-spoiler stuff and only reflects the state of affairs at the start of the first book/season. People die, ascend, descend, or trade places in some way all the time.
Game of thrones is the story of people trying to gain power and reign the main kingdom on the iron throne, a throne that seats the king of "The Seven Kingdoms" and is notoriously difficult to actually sit on. The name "Seven Kingdoms" is explained later in the story but it covers a big chunk of the entirely fictive world. There are few key places of this world in the first book. In the book each chapter is told in first person view of one of a handful of characters.
The kingdom could be said to start at "Kings Landing", a central place in the kingdom. The name is explained in back story, I won't go into it here. It contains the Red Castle, the Iron Throne, and is where the king lives. The kingdom then has a north, east, south, and west (obviously), and are referred to as such. The north has a huge (no I mean huuuge) wall to the north of it. To the even norther of that wall is a forest, which is not part of the kingdom and which is supposedly evil.
The Wall houses the Men of the Nights Watch. These men, male only, can be considered a neutral faction overall whose only task is to defend the Wall from its north. The men are sworn fealty to their brothers and once they do can never return to their old lives (doing so is treason and gets you killed). They are nicknamed the Black Crows. Once somebody joins the wall, usually not by choice, they pretty much default their old life. It's like dying without the dying part :) Well, not yet anyways. (There's an interesting philosophical notion to be found here, kind of relates to solipsism...)
In the north, below the wall, rules the Stark family. The Starks play an extended role in the first book and nearly all characters are covered in detail. They're deemed an honest and loyal family. Their line is "Winter is coming", referring to the north being the coldest part of the kingdom by far and only getting worse during winter season. As it happens, the past seven (or more?) years were summer and they are expecting winter to set in soon. They're also expecting it to be bad, as usual. Their line is much older than that though so for us viewers that's merely a coincidence. "Winter is coming" kind of caught on as a catch phrase for the show.
Kings Landing covers the stories of a few families, like the Lanisters, which are a rich family by nature. The current Queen is a Lanister. This family is deemed rich though deceitful. However deceitful, they will "always pay their debts". A line they'll frequently boast but do seem to take to heart. My favorite character by far is Tyrion, a dwarf with an exceptional speech stat. Combined with the influential power of his family he's capable of being very up front in an otherwise sea-lion-esque courteous setting which leads to some hilarious scenes.
Kings landing also houses "the hand of the king". The name kind of implies it, but he's the right hand of the king and is said to rule in his place. The king doesn't really rule as he's too busy with other things. Or so they say. The king also has a small council. The council seems to contain a bunch of random people, though at least one of them concerns itself with money, one with war, and one with communication.
Of course Kings Landing houses the king too, which is of house Baratheon (and he's married to a Lanister). They're also a good family though I suppose less so than the Starks. They do have a good bond with the Starks as will be explained in the back story early on.
The east contains house Arryn, who's Lord is the hand of the king and who's Lady is a sister of Lady Stark. Oh lords and lady's, you'd best get used to that sort of thing (or read up on it). It's castle is super high on a mountain or something.
To the south west are the Tyrells. They have a lot of farming going on there.
Then there's Greyjoy. It's hard to describe this family that lives on harsh islands. The backstory talks about a "Greyjoy rebellion", I'll leave it at that. Lord Greyjoy's son is a Stark ward (which is code for "hostage" in certain contexts).
To the south is house Martell. They're the only part of the kingdom where the first born inherits stuff, instead of the first born _male_ like in the rest of the kingdom. As it happens with medieval times, feminism hasn't really kicked in yet. There's also some more interesting backstory about the south part of the world I won't cover here.
That leaves us with house Targaryan. No backstory I can explain here, but their story line (mainly) happens outside of the kingdom. There's a son who wants to claim the throne. He has a (beautiful) sister. Oh and he's kind of an asshole.
The show/book tells the story of various people who want to sit in the throne, or people helping in this. It sets in a medieval setting with knights and swords and Lords and Ladies. There's courtesies and various castes from the high born to the low born. The story jumps geographically all around the kingdom but mainly follows abouuuut uhm, six sub stories or so. Of course this differs as story lines split, join, or wither when the main/only person dies. The story begins with a Wall, a Stark, a Targaryan, and a Kings Landing story line. All story lines intertwine in one way or the other, eventually. Usually in multiple ways. That much I can tell you :) The book helps in identifying how exactly they intertwine.
The story is quite explicit, gory, and sexual in general. I guess those were the medieval times, or so they have us believe... The tv show is quite explicit in depicting this. Probably even more so than the book. I guess they know what their viewers want, eh.
So let me stop here. I'll close by saying I like the show. The main selling point for me is the overall explicitness and the uncertainty of each character survival. I'm going to read the books and watch whatever show is coming. Obviously this is a story with no ending. There is no grand-arch main character role to end it. Well unless they nuke the entire planet to smithereens. Would not put that above them but since there's at least one more book to cover you can probably distill that _that_ hasn't happened so far. This means that new books can be written at any point, just like Zelda games can still be written. Although Zelda story lines don't really make s.. okay I digress.
The story lines are quite intricate. The book clarifies a large number of reasonings and links certain events together with greater detail than the tv show does. Still I'd recommend watching the show for a bit first because a: it's much faster than reading the books, and b: it gives you a good "medieval" imaginative feeling when reading the books. I also feel like certain characters from the book pop much better in the tv show (Tyrion, Cersei, The Mountain) which makes the reading the books so much more pleasant. OTOH who knows what I would have thought of these characters if I had read the book first. At least this way I got to read the text with medieval voices and articulation in mind :)
What are you waiting for?