“Cosmography” sounds like the kind of thing a pair of fantasy authors would make up, but it’s absolutely real.

When Alyc and I first came up with the concept for the Sea Beyond duology, our pitch to our editor handwaved something about a “Guild of Cartographers” being the ones behind the mapping and colonization of the Sea Beyond. Early in our research, though, Alyc — looking for resources on the cartography of the era — came across a book that had a profound effect on our plans: María M. Portuo...