Would a book about fictional geography go in the system or would that be in fiction?
Books about drawing maps for RPGs have been filed under 743.87 (how to draw fantasy) and 794.8 (video games). Geography itself is filed under the 910s, with 912.99 and 919.9 dedicated to extraterrestrial worlds, though those sections are more about the Moon and Mars and exoplanets rather then fictional worlds. Some books on fictional geography have been filed under 917 for travel guides or the 800s for literary analysis ("The Complete Guide to Middle-Earth" is under 823.912 for 20th century English lit). It would only go in the fiction section if it were a novel or a story about the fictional place.