Uncle Base is an expansion that adds basebuilding to FIST. It's also a direct continuation of the adventure in Rations #2 in the mainline game.
The PDF is 2 pages, with cleanly organized text, some nice graphics, and an action-movie-y feel.
Contents-wise, Uncle Base leans in on the Metal Gear overtones of FIST and gives the players their own outpost to develop. Missions now award money and construction crates. Crates can be used to build base facilities. Money can be used to buy crates or other rewards. The PCs can also recruit new agents to the base and dispatch them on missions.
Everything in this supplement feels rewarding to include, and growing the base gives FIST a Blades-In-The-Dark-esque gameloop. For players that like to chase the dopamine of progression, this is a VERY good add-on for your FIST game, but it's also just a nice backbone of structure for any FIST campaign.
Overall, this is a short module, but it's packed full of content. If you're interested in running FIST as a campaign---or even as just a regular series of one-offs---I'd strongly recommend including Uncle Base in your material.