The Codex “Guide To” series are not game books, but are rather historical encyclopedias of substantial length, very carefully and exhaustively researched, which are focused on a particular region of the world during a particular time. The first entry in the series, Codex Guide to the medieval Baltic Volume I, is focused on the Southern Baltic region of Europe, and more broadly on Central Europe in general, all during the late medieval period.
Codex Guide to the Medieval Baltic Volume 1 is available as a PDF and on paperback!
Volume II which covers different aspects of life in the same region, is not yet available but is scheduled for release by Christmas 2020.