You are going to absolutely LOVE this one!
I loved it mostly because everything that happened so far has consequences in this novel. Up until now I felt like the books have been telling stories, but the events and actions have been independent of each other. Sure, they are sequential, and certainly there are links between the causes and effects, but in Fortunes of War, which begins on the eve of the War of 1812, O'Brian locks everything up tight. And in some really nice ways: some characters return, some characters are new, but are associated with old ones, and we have some new locations, too. And a lot of surprise turns that frustrate Jack and add a lot to Stephen's story. In the some of the earlier books I found myself merely enduring Stephen's story, but on FoW, man oh man, it gets good! Spy stuff!
I also especially liked the way O'Brian highlighted some of the conflicts I hadn't thought of: the new Navy and the old both in technology (the guns, for example), the clothes, and the character/attitude of the officers. Jack's old school and at the start we meet another old school captain and then, near the end of the book, we met a new school captain. There are a few others, but I don't want to tip into spoiler territory.
There are some good sea battles, but most of the novel takes place on shore. That might sound boring, but Stephen's troubles kept me so stressed out I had to keep reading.
And I see that the next book, The Surgeon's Mate, takes place directly after this one. In truth, I've started it, but I'm still in the first dozen or so pages, fighting through the annoying, but necessary review that begins sequels. But I expect the dovetailing to continue, at least until we get to those books that don't exactly fit into real life's timeline