A book lung is a rudimentary type of lung found in arachnids, such as scorpions and spiders; horseshoe crabs also possess book lungs. They are not evolutionarily related to mammalian lungs. Their name describes their structure, as they are "folded" like a book.

The unfolded "pages" (the plates) of the book lung are filled with hemolymph (the arthropod blood). The unfolding maximizes the surface exposed to air, and thereby the chances of gas exchange with the environment. In most species, no motion of the plates is required to facilitate this kind of breathing.