Subtitled "A Lexicon and Companion to the Complete Seafaring Tales of Patrick O'Brian," this is an absolutely marvelous book, the Third Edition of which includes references to all twenty of the Aubrey-Maturin novels. It scores high in the first test given any alphabetically organized reference book, viz., in looking up an entry, ... There's a wide variety of nautical jargon, period medical terminology, the characters' references to natural history and music, and the foreign words and phrases that crop up in the novels. O'Brian describes a large number of real personages, too, all of whom are succinctly biographed. There's also a pretty detailed timeline for the period 1793-1818, a narrative essay on the ins and outs of the Napoleonic wars, a most illuminating discussion of naval medicine and surgery in Maturin's day, and a nice series of period illustrations of ships and boats for those who can't tell a frigate from a corvette, nor a barge from a launch. This is definitely a book to keep at hand while you work your way through the series.