HaroldTheHobbit
Hero
For the impact it’s had on my gaming, probably Runequest 2ed core.
My first thought was for the old Flying Buffalo Citybooks series.
I will say that Aaron Allston's Strike Force, mentioned in the original post, is worth at least a honorable mention. I'll also put down some of the D&D Mystara gazetteers as honorable mentions, in particular The Grand Duchy of Karameikos, The Elves of Alfheim, and The Dwarves of Rockholm. (And yeah, two of those three were also written by Aaron Allston. Maybe a pattern here.)
1e DMG - Something about everything, even if you wouldn't or shouldn't use it.