Maybe the librarian used to be a butcher and still has the meat scented hands with he/she uses to touch the books, ok out of the joking part, I think that maybe the books got some scent on it that he likes and so he chews on them. Get a library book and try scenting it and see his reaction, should be different as I think. I agree with Karen and with Taz_Zoee. Keep the books on high places or in closed places.