With the Lightnings, by David Drake.
I have only had experience with David Drake through his fantasy novels (Lord of the Isles, etc.) but I know he is best known for military sf.
This was another find from the Baen Free Library. I don’t know what the title really has to do with the book, because there isn’t anything to do with Lightnings, but there you go. This book has been compared to the Aubrey-Maturin series, and I sort of see it, but it isn’t really like those at all except superficially.
The two protagonists, an exiled sharp-shooter librarian turned spy (Adele Mundy) and a lady’s man lieutenant with a passion for nature (Lt. Daniel Leary) should hate each other, as Leary’s father ordered the massacre of Adele’s family. The two end up unlikely allies, and then friends when the planet they are on becomes the focus for a bloody coup.
This on the surface is pure space opera, but it is the combination of Leary and Mundy that makes it work. I really liked Adele’s character, she is smart, practical, comes through in a pinch, a crack shot with a pistol and she loves looking things up. Leary is an idea man, untried in combat despite his bravado, and always is finding some species of flora or fauna that he wants looked up, so they are a great team. I see there are a few books in the series, so I will probably pick them up.