Joke about how southerners are meant to be 'dumb', 'inbred' and 'hick-ish'.
Were the boys more turned on by the naked chick on the front, the lesbian sex inside or the dog on the Amazon page?
Seriously though, it's not THAT bad.
The book could have been placed in the wrong section - not the library's fault. If the father/the kids are going to be that worked up about it, then they need to realise that it was the kid's fault - they should have known that it was not a book about the military.
If it was me in the kid's situation - that book might need to be renewed.