The books are already modified by the libraries adding various ownership markings. The thieves will of course remove those markings before selling the books on,

The books are already modified by the libraries adding various ownership markings. The thieves will of course remove those markings before selling the books on, which causes far more and more obvious damage than the markings themselves. The benefit of something like RFID tags is that the thieves would need to remove the tags on-site to avoid detection, rather than at their leisure.