The Call of the Wild by Jack London
Julie of the Wolves by Jean Craighead George
Animal Farm by George Orwell
Captains Courageous by Rudyard Kipling
Where the Red Fern Grows by Wilson Rawls
Treasure Island by Robert Louis Stevenson
The Little Prince by Antoine de Saint-Exupery
The Hobbit by J.R.R. Tolkien
Pawn of Prophecy by David Eddings
The Ones Who Walk Away From Omelas by Ursula K. LeGuin (a short story, find it in a sci-fi stories collection)
Nightfall by Isaac Asimov (same, voted the best sci-fi story ever)
Also check out Star Wars books, nonfiction books on subjects that interest you (get help at the library so you know how to find good stuff), and the young adult section. The humor section can also be a good way to ease into reading. Keep in mind that there's nothing wrong with reading books written for young people (many of the best books are accessible to both adults and children). Just because a book is more complex doesn't mean it's better, but expanding your horizons is good too. I used to hate nonfiction when I was a kid, but that was mostly because I never really tried it. Now I really enjoy it.