I watched the first season, then read the first three books. I was impressed how closely the first series followed the book, but with season 2 and 3 there are a lot more variations around vs the books. In a sense it is good because it makes it trickier to anticipate events and both manage to keep an identity of their own. I really wonder where season 3 is going to end?
The books are kind of tricky to read being written from different perspectives to further the story line. The second book is especially tough to figure out who is who and what is going on etc. It was quite a departure from where the first book left. Later on I couldn't put it down, you always want to know whats going to happen next! :)