Other forms: biblioklepts
Mysterious gaps in your collection of romance or sci-fi novels might mean a friend borrowed a few without asking, or it might mean a dastardly biblioklept, a book thief, is on the loose.
Biblioklept comes from the Greek roots biblio, meaning "book," and klept, meaning "thief." From klept, we also get the word kleptomania, a compulsion to steal regardless of financial need. When that compulsion is specifically to steal books, it's called bibliokleptomania. Interestingly, a habitual biblioklept is probably also a bibliophile, one who loves books.