Sea of Words : A Lexicon and Companion to the Complete Seafaring Tales of Patrick O'Brian by J. Worth Estes, Dean King and John B. Hattendorf (2000, Trade Paperback)
If you plan to read the Aubrey-Maturin novels, grab this!!!
I haven't read much of the Aubrey-Maturin novels at this point, only just now nearing the end of the fourth. But this book has already helped me make sense of a good few naval and medical terms, thanks to it's large glossary, and I imagine it will help me many more times before I have completed the series. It also contains articles that give an overview of the wider conflicts going on between nations during the events of the novels, about medicine as it was practiced then, information on important institutions and key historical figures, and ship diagrams and pictures of various types of ship. I would say this is something you should purchase as soon as you know you want to keep reading the novels, which I think will be before too long. You can still read and greatly enjoy the series (well, the first four books anyway), without knowing much at all about how sailing and victory are achieved on the high seas, but if you want to develop a reasonable understanding and gain even more enjoyment from these sublime novels, I don't know if that's possible just reading the books themselves - a flaw perhaps?
In summation, this is a fantastic companion book to have, and due to it's nature as a compendium of naval knowledge, an interesting read in and of itself - get a copy as soon as you intend to stay aboard for the long haul!