Lost Library is a Mage type Trap Room introduced in Boss Monster: The Next Level
This card is best built over a room with an uncover effect. This card can be built, and saved for later, when the player would need a Spell card.
The player could choose to discard a Spell card that was in their hand prior to activating this card's ability.
It was noted in an update by The Brothers that this card works a lot like Dark Laboratory, which it in fact does, albeit with a different activation requirement.