Due to my interest in type theory (and higher type theory), I have found that learning about sheaves might be useful (for, e.g., sheaf models of type theories). There is Kashiwara and Schapira's Categories and Sheaves, but that seems way out of reach for me at the moment.
My background includes some category theory and a little bit of algebraic topology, and I'll probably have to read more of both. However, what else do I need? Perhaps learn some algebraic geometry, or higher category theory?