My sims will take books out of the bookcase, but they won't put the book back. It doesn't matter if I click the book or the bookcase.

If the options to "Clean up books" or "Put away book" are grayed out, the game may not be recognizing bookshelves on the lot. This could be due to the bookshelf being outdated/broken cc, or other broken mods affecting the action.

If the options are available, but your sims shows a routing error when trying to complete the task (waving their hands and having an x with footprints in their thought bubble), the books or bookshelves may be inaccessible to the sim. Try moving the book/bookshelf or moving nearby objects out of the way.

If the options are available, but your sims just don't do it or cancels the action, the problem is most likely broken cc or mods.