Tome of Spell-stealing

This large, obviously magical book is sometimes found in treasure troves. Characters looking it over will assume it is some kind of spellbook or even holy book. In fact, it is a clever trap.

If a mage or cleric reads this book, even just flipping through it, they immediately lose one random spell of the highest level they have currently memorized. The effect is as if they had cast the spell, so they cannot immediately just memorize another spell. If they continue reading, they will continue to lose spells at the rate of one spell per round of reading.

The book can be burned or otherwise destroyed, just as a normal book can be. Some spellcasters, after realizing what the book is, will keep it among their other spellcasting materials, to punish any would-be thieves.