Nightwing and Robin are on the cover of Nightwing, Vol. 3: Nightwing Must Die, and both of them wear masks.
Book themes for Saturnalia: The god Saturn has a planet named after him; read any work of science fiction that takes place in space. –OR– Read a book celebrating free speech. –OR– A book revolving around a very large party, or ball, or festival, –OR– a book with a mask or masks on the cover. –OR– a story where roles are reversed.