OK, about the stickers in Watchtower pubs idea: EXCELLENT!
But in the case of the used bookstore with security cameras: the next time you go there take a survey of where the cameras are, which way they are aimed, and where monitoring blind-spots might be. It is unlikely that they have 100% area coverage. Once you know where the blindspots are on subsequent visits you can select a few books, and put a sticker in just one of them that way, if you're "caught," you can deny that you put the sticker there. Just do not have a pocketful of additional stickers on you when you go into the store, right?