Though most of the novels can stand alone as far as the plot goes, they are best read in order so that the development of the characters can be properly appreciated. In Master and Commander, the first volume in the series, Aubrey, then a lieutenant, meets Maturin at a concert in Port Mahon (a shared passion for music will be one of the mainstays of their friendship). When he is subsequently appointed commander of a 14 gun brig he takes Maturin along as surgeon. They roam the Western Mediterranean attacking merchant shipping and, in an extraordinary but historical action, take a 32 gun Spanish frigate in a single-ship action. The book closes with the battle of Algeciras.
Eighteen books is a lot, but O'Brian seems to have no trouble finding new material after a quarter of a century. The most recent volume, The Yellow Admiral, features a prize fight, political machinations over the enclosure of a common, and Aubrey's fears, as he approaches the top of the captains list, that he will be made admiral without a station, the "yellow admiral" of the title. On top of all of this, Aubrey's marriage is in trouble. There is no climactic naval action, but we are treated to a retelling of Pellew's destruction of the Droits de l'Homme and a picture of the Brest blockade in the closing days of the war.
The series as a whole encompasses too much to be more than hinted at: battles of all kinds, both against the enemy and against storms, ice, calms, and all the other perils of life afloat; journeys around the world and stopovers in ports on every continent, from New York to Bombay, from Freetown to Port Jackson; romantic entanglements and marriages (and Aubrey's Sophie and Maturin's Diana are worlds in themselves); delicate diplomatic missions and cloak and dagger skullduggery; medical problems and excursions in natural history (Maturin is forever being torn from investigation of taxonomic wonders by the exigencies of naval service); engagements with the often mysterious workings of politics and the law; and a wealth of memorable minor characters, some who appear just once and others who reappear regularly.
O'Brian does take a few liberties with his history. A form of time dilation stretches 1812 out so as to fit in several extra years of war. The choicest naval actions are appropriated for his heroes (and where American readers might baulk at this, in the frigate actions of the War of 1812, they turn up as guests on board the participating ships). The machinations necessary to have Aubrey and Maturin move continually in and out of debt seem a little implausible, even allowing for the vagaries of prize money. And O'Brian even has the chutzpah to invent a species of turtle for Maturin to name after Aubrey!
These are very minor and deliberate liberties, however. O'Brian's feel for the language, customs, atmosphere, and detail of his period seems flawless. He is as much at home with politics, people, science, or indeed anything else as with matters naval. (O'Brian has also written a biography of Joseph Banks.) When he ventures a little into Austen or Heyer territory, for example, in the meeting of Aubrey and Maturin with their future wives (in The Post Captain, during the Peace of Amiens), one feels that if he had wanted to he could have written a first-rate period romance. In these novels O'Brian has produced not just great naval fiction, but outstanding historical fiction.