This is a magical book. On facing pages are paintings of monsters and a spell written in invisible ink, which can be revealed only by applying some blood, squeezed from the heart of the creature illustrated.
- Red Dragon
you could choose spells related to the monster (Polymorph other Chimera, for Fly for Pegasus, Stone to Flesh for the Gorgon, etc.) but you can substitute any spell, with a level no more than 1/2 the monster’s HD. These spells should not be otherwise available.
Alternately, the book sits at the entrance of a dungeon. Soaking the image with the right type of blood will open the portal to a deeper level, or open areas of levels already revealed.