People have conversations with each other, so why can't books?
Intertextuality is a conversation between books. Intertextual books reference other pieces of writing, and use those references to make meaning. Have a look at our discussion of intertextuality in Rebecca Stead's When You Reach Me for more.
But intertextuality isn't just for books, so keep a weather eye out for it when you're watching your next episode of Community.