Deep, Dark by Jonathan Maberry (Joe Ledger #1.5)
A short intermediate story set between book 1 and 2, but entirely standalone. It's only about an hour long, as an audiobook. Joe and two of his team, Bunny and Top, head out to put down what they're told is a terrorist infiltration and turns out to be something entirely different.
I read book 1 (Patient Zero) quite a while ago and really really liked it, but somehow the rest of the series fell into a crevasse on Mount TBR. So this was a really good way to fall back into like with the characters and the style, without going back and re-reading all of Patient Zero. It's gritty, and dark, and Joe is the same honourable hardass I remembered.
I think this is actually strong enough to stand on it's own, as an introduction to anyone curious about the series, it's a short time investment. It's a little lighter on humour than Patient Zero was though, although it has a couple of moments.