The Codex Guide to the Medieval Baltic is still the definitive gazetteer-style resource for people interested in running historical or historically-adjacent games. The writing is extremely accessible, well-researched and organized in a 'desk reference' style that covers a very broad range of questions. Carefully-chosen art provides additional context; almost every page has images, and many have multiple, all with accompanying explanations. The bibliography is like a free reading list to help you find more granular information.

This volume has extensive coverage of law, religion and academia in the late medieval world, and provides ample material to fill in narrative gaps in campaign settings that are derived from the 15th century in Europe. I've used both books in the series as resources to get quick and effective answers to player's (or my own) questions on these topics, to great effect.

Like most Codex Integrum material, these books are an essential informational and system-agnostic supplement for people writing and playing fiction in the time between the Dark Ages and Renaissance. The 'medieval times' of your mind's eye is probably much closer to the 15th century (vice 13th, 12th, etc.) than you might believe - and at the same time, full of much more intrigue, drama and action than a cursory glance would tell you.