Kinmaker is an expansion for FIST that increases the overall power level and supernatural volatility of the game by adding psychic abilities in the vein of Akira and Firestarter.
The PDF is 23 pages, with a solid and easily readable layout. The vibe is more sleek modern supernatural and less old military technical handbook, but it's consistent and nicely atmospheric.
In Kinmaker, using supernatural powers generates Heat. Heat causes catastrophic effects if it gets too high, but decays by a big chunk at the end of every session.
Powers are unique to this book, but are gained by taking Traits from core or from Kinmaker. Foes can also use powers, and there's a neat multi-page bestiary included, with multiple possible powers for each foe.
Powers are individually statted out, and each include three different levels of use with corresponding Heat costs, plus a bonus for mastering them.
While not part of the book's central theme, there's also a weapon rarity mechanic, weapon attachments, and some other peripheral rules that add more of a modern looter shooter feel to the core gameplay.
Overall, if you want to focus on pulpy psionic espionage, I'd definitely add this to your game of FIST. The mechanics are fun to fiddle with, the themes are solid, and there's a lot of player and GM-facing content in this meaty supplement.