Ten People You Meet In The Undergarden is an extremely high quality set of NPCs/PCs/at least one location that you can use easily with Troika, or a little bit more effort-involving-ly with any other system. The overall tone is weird and surreal science-fantasy, so they might not plop into a serious and realistic game without changing the setting, but they'll fit pretty naturally into Hill Cantons, Into The Odd, or maybe Dark Souls.
Structurally, Ten People is 14 pages (a lot of which are double-page spreads, so technically 25 pages in print,) and it has off-the-charts good layout and illustration.
The writing is *really strong* too, but it's not really oriented towards being game-writing. It doesn't distill down and give the GM the actionable material in a concise and easily accessible format. Instead, it's more like ten really good vignettes. You'll get a great understanding of each character, but it takes time to read.
Mechanically, the NPCs all feel like interesting bits of the world to encounter. Some feel like campaign hooks. Some are just solid one-hour obstacles. Some are more like optional color, but there isn't a single one that I wouldn't be stoked to drop into a game as the GM.
Overall, for as narrow as this book's scope is, it's some of the best supplement content I've read. I highly recommend picking this up if you have Troika, or if you write/GM osr and have gotten creatively stuck.