At some point O'Brian decided this series would go on indefinitely. The structure of some of the books then became odd. There are some that don't end - they just stop. There's an obvious on-going, unresolved plot but - tough luck - you're gonna hafta wait for the next volume to get a resolution. This is one of them. It ends with a cliff-hanger (which some don't) and for some reason it's easier to handle then when a book just stops apparently arbitrarily.
So, thirteen books in and it's getting harder to find non-repetitive things for Aubrey and Maturin to do with each volume, yet O'Brian pulls it off again! Making Maturin not just a physician but an intelligence officer was a stroke of genius in this regard. It offers a much greater range of possible and plausible adventures than regular Royal Navy work could...and so we are off to the Pacific on a diplomatic mission with clandestine additional motivations. The best, most delightful part of this volume is an almost complete aside from the main plot, however: Maturin indulges his interest in natural history by visiting a volcanic island where Orangutans abound.
Back to the cliff-hanger. I have not had so urgent a desire to read the next in this series since probably somewhere in the first five volumes. I think I overdosed for a while and I also think the quality varies somewhat between volumes in this series, but this one is the best since the tenth, at least.