Thank you to everyone playing The Night Librarian, and a special thanks to the player who reported this one.
Fixed
Chapter 4, Puzzle 5 ("Sort and File"): the second part would not register a correct solution, leaving the puzzle unable to progress even when the books were arranged correctly. It now completes as intended.
Your save is safe. This update does not affect existing progress. If you were stuck on this puzzle, load your game, arrange the shelf again, and it will complete and let you continue.
If you run into anything else, please leave a comment or a review and let me know. I read all of them.