Where to find the stone mask in Vampire Survivors

Level up to level 20 in the Madwood to unlock the library. Go to a new location, run along the top edge of the map to the right. After two and a half minutes, you will find a niche with a mask. Pick up and get a 10% bonus to farming coins.

If you run to the left, you can find an empty tome. Another useful item. Also in a niche at the top of the screen.