Ronin is an expansion for FIST that takes a more noir-y, spycraft-y angle.
The PDF is 9 pages, with clean, readable text, some great illustrations, and a high contrast, high white space approach to the layout. It looks sleek and bold and it's fun to scroll through.
Contents-wise, this expansion adds the Ronin Role, which lets you advance when you play up being a burned or burned out spy. It also adds a few related Traits and a faction of displaced bushi living on the margins of the modern world, plus some mechanical support for doing things during the downtime in between missions.
That said, Ronin feels very much like its own thing. Its inspiration from the '98 film of the same name, and its lower weird, lower supernatural tone make it a good match for an espionage game, but I got the feeling while reading it that it would shine even brighter as its own standalone system.
Overall, if you want to anchor your FIST game in the nuts and bolts of Soviet-era European espionage, or if you want a less fantastical and more noir tone, give Ronin a look.