I have been preparing to run two new 5e games, one that will be wilderness heavy and another that is in the Underdark. Two books instantly came to mind from decades of gaming, the first edition AD&D Survival Guides.
The Dungeoneer’s Survival Guide‘s greatest claim to fame is it introduced the Underdark. Prior to book the modules D1-2: Descent into the Depths of the Earth and D3: Vault of the Drow introduced the underground world, but only for this adventure series. The DSG named this location the Underdark and opened it up as a new adventuring location, which spawned all new campaigns and novels (heard of Drizzt?). So I knew I needed to read this book again for its Underdark info, but I also love it for its extensive rules and explanation text for underground adventuring such as spelunking, air supply, cave-ins, mining, underground waterways, and new equipment. The follow up book, the Wilderness Survival Guide, was not popular but still contains some great info on environmental effects, weather, flying and waterborne movement,beasts of burden, and natural hazards.
I am also very partial to the artwork on the covers of these books. Rather than typical high fantasy adventures showing heroes defeating (or at least challenging) their foes, this shows the more mundane experiences of adventurers struggling with their environment and the creatures who call it home.
Even if you do not play D&D, these books are great references to add depth to your fantasy campaigns and writing as they highlight unique elements to wilderness and underground environments and adventures. Reading these books again brought to mind other areas I should research in more depth for my upcoming D&D 5e campaigns, inspiring my upcoming campaigns. Though they are out of print, you can still acquire PDFs of the books at DriveThruRPG.