Thanks for commenting ^^
You should be able to enter (and leave) the storeroom at any time until you place the final book back. There are a few optional events that can only be done before the final book, but nothing that affects the storeroom... Did you try to interact with the book once you were on the chair? It wasn't an autorun event, you needed to use the action button to pick up the book as it was for the others.
Not being able to leave the storeroom is the most concerning as it's set to a simple conditional else branch. Can you remember if there was a message "I should put this book back before I leave," despite you not having one, or did nothing happen?
I know nothing can be done now, but I do have to admit that it was quite depressing to be disqualified so it'd be great if I could figure out what the problem was and make an update :)