The Hobbit by J. R. R. Tolkien. I think I was 10 when my mother put this book in my hands. I devoured it. I had already seen the 1977 cartoon by Rankin/Bass and the book was even better. You can check out part of the movie at youtube.
Eventually, I read the other three, along with the Silmarillion.
I was also taken with the early cartoon Flight of Dragons. You can watch part of it here!