I know it's stretching a bit to call the Jack Aubrey novels mysteries. Hell, it's stretching a lot, even allowing Dr. Maturin's status as a secret agent.
Still, since O'Brian's oeuvre has come up in the past, this seemed like a good category to post a comment in.
I recall getting into a friendly argument some time ago about whether O'Brian was better than C.S. Forester. I was a partisan for Forester and the Hornblower series. But I have to admit that, the more I get into the Aubrey series, the more I'm getting converted to the O'Brian side.
For one thing, he really does capture the writing style of the era, without seeming pedantic (as so many Jane Austen continuers do). For another, his depiction of the friendship between Jack and Stephen is much more engaging than the parallel one between Hornblower and Bush. For one thing, Stephen is more of a co-hero than a sidekick, and his status as a Ship's Surgeon places him somewhat outside the chain-of-command, so there's no need for the distance that must exist, by necessity, between Hornblower and Bush.
Finally, Jack strikes me, ultimately, as a more believable character than Hornblower, for all that he seems constructed, in some ways, to deliberately be the "anti-Hornblower." Much as Hornblower's self-doubts, sea-sickness, and discomfort around women he's attracted to are endearing, how likely is it that someone so beset by such doubts would be this successful in a military career.
Jack's being so absolutely comfortable in his own skin, so decisive, and so confidant of his own decisions, all are elements one is much more likely to associate with a successful career in the Armed Services.
I'm up to Desolation Island so far, and I'm truly looking forward to the rest of the series.