Since I've decided to do what the_Gneech is doing, I'm making the book a treatise on the "worlds under Miraboria" - ie now a historical text on 400 years-lost-realms under the current kingdom. The map is just one page in the book, and there will be several other places described that connect up from the surface world that they could choose to investigate, if for some reason they prefer not to sail the sunless sea...