My favorite book of all time is actually a series of books. The Sword of Truth series by Terry Goodkind (contains ten books in the entire series).
As for whether or not it "helped" me in any way...as with any good book, it always helps a person to escape from reality for a little while and immerse themselves in a fantasy world of vivid imagery, rich and detailed landscaping, and intricate character creation.
If an author writes so well that I can't put a book down, to me that's definitely "helpful." :)