Too much is different from the series from the books. I watched GoT season 1 before picking up the book and I learned so much more from the first book that reading later on you will not understand how it got there or why it's happening if you skipped the first book, let alone the second. Definitely start with book one.
Anyways.. it's a waste to not read a book you already have. It's like watching the first Harry Potter then refusing to read the book that's been gifted to you just because you "know how it ends". Then wondering what the hell is going on in book 2.