When the Tide Rises, David Drake. 2008, Baen Books.
This is the sixth novel in Drake's RCN series. I haven't read any of the previous books, but that didn't make much difference, as far as I can tell. Perhaps I missed some of the in-jokes or references, but that's not a big deal. It stands well on its own.
In the introduction, Drake cites the memoirs of Lord Cochrane as his source/inspiration for these books. This is the same memoir that Patrick O'Brian used for his Aubrey/Maturin books, so they bear a lot of similarity.
Commander Daniel Leary is in the employ of the Republic of Cinnabar Navy, and he's assigned to a planet that's declared independence from the Alliance, Cinnabar's enemy. He makes a stop at Diamondia, a planet under siege by Alliance forces, to check in with the man in charge there and see what the most recent intel is.
Adele Mundy, Leary's signals officer and librarian-hacker-spy, meets a young man who's the grandson of her mentor. His parents have just been killed by the head of the Alliance, and he wants her help. She brings him along on the trip, even though her sociopathic servant and bodyguard Tovera thinks it's a bad idea.
Once they arrive at Bagaria, Leary is enjoined to the Bagarian fleet, and he commands them in a few raids in Alliance space. There are crosses and double-crosses, and politicking, and hardcore librarian spying.
There are a lot of details about the ships and sailing, and I will admit I skimmed them. I found it odd that starships had literal sailing masts and rigging, but it fit the book itself. I'm not sure what the sails were made of (probably described in more detail in earlier books in the series), though I remember mention of it being very thin. (Nanomaterial?) I will say that it's disheartening to read about a ship with the name of your football club (which itself is named for a ship) being sunk a few hours before they're set to take the pitch. (Sports fans are a superstitious lot. We won, btb, for the first time this season.)
There's a lot of adventure, and I really liked the hardcore traumapast librarian. If you read the Aubrey/Maturin books (Master and Commander, etc) and thought, "You know what these books need? Spaceships," the RCN series is for you. If you like Bujold's Vorkosigan books and don't mind fine attention to sailing detail, you might enjoy them, too.
The first three books are available at the Baen Free Library, beginning with With the Lightnings.