"1984" made me a fan of dystopian/speculative fiction.
"Eye of the needle" made me a fan of world war/ espionage thrillers.
"The Gulag Archipelago" made me a fan of historical non-fiction.
"Pillars of the earth" made me a fan of historical fiction.
On the other hand, I didn't encounter a book in the past that would have made me a fan of the epic fantasy genre. Sure, I read fantasy; I have had my fills of Tolkiens and Rowlings (and these books are great and I like them) but I needed one book from the genre which would have made me crave for more.
This is that book.