Any kind of classic, if you're into that. (You don't have to get the original because it might be kind of boring but then again you might not want the Dalmatian one with the 7-inch text)
I like Wereworld but nobody else I know has even heard of it.
I also recommend anything by Kathryn Lasky.
I didn't really dig the Guardian Herd but if you're into fantasy that's for you, although I find it needlessly gory.
As far as mysteries go, I LOVED the FunJungle books. They don't really specify which is the original but I think it's Belly Up. It's a trilogy, though, which is mildly disappointing because I was kind of hoping for more.
If you'd like me to give more suggestions on anything, specify the genre, pleae.