Adult books are the books that have been aimed at older people (which is obvious to anyone who can open their eyes). Harry Potter may be a book that everyone can read, but the fact remains that it is aimed at kids, not young kids, but still young readers. There's no problem with that, but I still think that people should realise that there is more to the fantasy realm than a 15 year wizard fighting against the same person (and always winning) in each and every book...
I haven't read the latest book, and I hope that I never will.