There isn't just one, that would be silly. I think it is often a pattern of reading- many good or great books in a row-that turn a casual reader into a voracious one. Besides, a book that turns a child into a reader is not the same thing as one that captures an adult. The book that makes your heart skip a beat in your 20s is not going to, generally, have that effect on someone 30 years older. For the sake of doing more than lightly ranting, I'll share a book that definitely influenced my reading for years.
Still one of my all-time favorites, Practical Magic blends real life and fairy tale sensibilities in the perfect balance. It bridges a genre gap that would likely capture many readers of different types, and I have loaned my copy to several people in an effort to convert them.