He's written several, with co-author Lin Oliver. The first series is called Hank Zipzer about a dyslexic boy, which is based on Winkler's own experiences. His newer series, also with Lin Oliver, is called Ghost Buddy, about a kid who ends up befriending a wacky kid-ghost.
I facilitated a book signing with both authors and Winkler was very candid - and very funny - about his dyslexia. A real nice guy in person ... and he's got comedic timing down to an art form.