Game of Thrones is as most of the other good TV-shows out there based on a book-series called "A Song Of Ice And Fire". And if you don't want to read the books, and thereby risk to spoil the series, here is some alternatives!
My personal top three epic fantasy novels:
- Malazan Books of the Fallen by Steven Erikson
- The Prince of Nothing series by Scott Bakker
- Hyperion Cantos by Dan Simmons
If you love reading book-series, you can check out: "Wheel of time", "Mistborn series", "Black Company Chronicles" and "the Darkness-series".