You can drop a book in Clean the Library on PC by pressing Q while holding it. On controller, the game listing says B/O drops the book, and E or click picks up or places a book.

If it’s not dropping

  • Make sure you’re actually holding the book and not just standing near it.
  • Try pressing Q once, not holding it down.
  • If you’re on controller, use the listed drop button from the controls menu.

Useful detail

The game’s page also says books can be picked up, placed, and switched, which is why some players think they “can’t drop” them when they’re just cycling or placing the top book instead.

Quick answer

  • PC: Q
  • Controller: B/O
  • Pickup / place: E or click

TL;DR

If you’re already holding the book, hit Q on PC to drop it; that’s the standard control shown for the game.