Sometimes it would seem to reboot but would always come back and get stuck on Opening Book. And then the Reader would start a sort of reboot sequence by its own, always getting stuck at Opening Book.
When my prs-505 restarts, which it does with some books, it returns to the home page after a soft reset. Are you going back to the 'faulty' book and trying to re-open it, or does the reader try to do this on its own?
Usually, this is a battery issue. You may have to contact Sony support, a bla suggested.