In the 20th century, the British Isles have produced many great series of fictions in which one can follow a group of familiar and often endearing characters in their adventures. Many like Tolkein's Lord of the Rings or J.K. Rowling's Harry Potter are fantasy worlds of no small entertainments and fancy. Others like Agatha Christies Hercule Poirot or Ian Fleming's James Bond have a mix of murder and sinister spying intrique (see Agatha Christie's The Big 4 for a Hercule Poirot in the spy game). Thus Patrick O'Brian's historical novels set in the Napoleonic War era and featuring Captain Jack Aubrey and his Boat's Doctor, Steven Maturin are a departure from the the novel norm.
And as the stories are almost always set at sea with all physical aspects of sailing being the beckoning backdrop of every story. And Patrick O'Brian is far from romanticising the often harsh surrounds, relentless work and sometimes brutal life aboard a British Sailing Man-of-War. What makes these stories so interesting is the subplots of diplomatic and spying intrigue that Steven Maturin is carrying out on each mission. Also the books convey the spirit of the times as France and England now compete most directly for dominance of the seas and the emerging sea trading routes and patters.
These books with their naval warfare nature and spy subplots would appear to be for manly Men only reading until you consider the constant role that strong women play in any number of the stories. Also the sustaining interest of both jack Aubrey and Steven Maturin in various aspects of the rapidly emerging Natural Sciences and the musical arts provides a culture backdrop that is both unexpected and helps to sustain interest. Finally, the emergence of both medicine and incipient psychology from almost primitive blood and gore or dire superstition to rational natural sciences provides many an adventure and historical insight.
But it is the friendship of the Captain and his Doctor that makes this story so fascinating and of sustaining interest. They are buffeted not just by war and the challenge of keeping a sailing ship in top shape; but also various philosophical divergences that they must accommodate in order to work effectively together that adds to the often South Seas Sailing Spice of these stories.
Let me highly recommend reading the books in order as described here. Even better, if you have a long commute, the audio readings of the books, though a bit more dear (about double the price of the hardcover books) provide top-tier theater readings of the stories - more than well worth the price of admission.