Die Twenty is a bestiary for FIST that adds explicitly fantasy rpg monsters to the roster of things the agents can be called to combat.
The PDF is 4 pages, layed out in a perfect mirror of the core book, and is clean and easy to use.
Contents-wise, everything here is extremely flavorful. It would've been easy to just restat DnD critters to FIST, but Die Twenty goes the extra mile and integrates them into the mundane 1980s. The merchant princess is a corporate heir. The dullhan packs a submachine gun. The halflings break bad.
There's also some *really good* storytelling in the margins about a Satanic Panic style demon outbreak. The Lake Geneva Exclusion Zone is a stellar framework for a campaign, and your group can move fast and break things in ways a big IP like Stranger Things can't.
Overall, if you want more monsters for FIST, you want this.