Wait, so this is a 5E version of the AD&D First Edition Dungeoneer's Survival Guide (affiliate link), the supplement which introduced the term "Underdark" to D&D, written by that book's author? I'll admit, that's pretty impressive!
Awesome! The DSG and WSG were two of my favorite rulebooks in AD&D.
Personally, I am VERY surprised WotC hasn't produced expanded rules for such things, combat, etc. for the players who enjoy 5E but want more rules in place. I know A5E (Level Up, or whatever) is doing a lot--but it seems like a missed opportunity for WotC. shrug