I have been noticing from a few librarians in my trading hall village, once I buy the first enchanted book listed in their trade list, the book in the output slot will switch with one of the next enchanted books in the list. This is a little inconvenient since I sometimes buy multiple copies of the same book, and then accidentally buy the wrong book when it gets switched out.

Also, I noticed that one librarian has the same book twice, with the same price.