I read the book a few months back, I was rather disappointed. I just recently seen the movie, which had a completely different ending than the book. The movie failed to impress me any more than the book did, altho Judge the dog was really cute.
As for a grade level...I'd say the book is appropriate for those in grade 9 and up.
Abigail~ APBT~ 2
Jersey ~ APBT~5(ish)
Kato ~Rottie~ 5&1/2
Wendellan~ Feline~ 10(ish)
"Da-Goob"~ APBT/Dogo x (5-6ish)
"My Pitbulls are better behaved than your kids!" (my favorite bumper sticker)