The classic of classics, a 3-book world that came into being when a linguist thought of his own language. It's technically 3 volumes, but since they were meant to be one book, and only split into three because of publishing, I'm going to go with one prompt here.

I do not recommend you begin with the Lord of the Rings, because it is written classically. The writing style takes some getting used to! Modern fantasy is on this list too, and might be a better ice-breaker for someone unaccustomed to heavy prose.