Fighters fear the Rust Monsters but spellcasters of every type fear the Giant Silverfish. These insects crave and can quickly consume scrolls and spell books.

The Giant Silverfish
Armor Class: 9 [10]
Hit Dice: 1 HD
Attacks: None
Saving Throw: 18
Special: Eats Paper, Spells
Move: 13
Eats Paper: A giant silverfish can consume one scroll or one page of a spell book per round. This includes things like treasure maps or other valuable documents but the creature will prefer magical texts.
Spells: When a Giant Silverfish consumes a magic scroll, it makes a Saving Throw. If successful the beast can instinctively cast that spell once as if it were able to read the scroll. It can do this for either magic-user or cleric scrolls. Any Giant Silverfish encountered will already have consumed 1d6 spells from levels 1 to 4. determined randomly.