It's my hc that there was a storage closet in the Hogwarts library containing 40 copies of The Monster Book of Monsters because every student in Harry's class refused to take theirs home at the end of Third Year.

During the Battle of Hogwarts, Irma Pince heroically defended the library by opening the closet, unbuckling the restraints on the books, and hurling the books at Death Eaters in the hallway.

It's going into the fic. :)