Anyone interested in the any aspect of the Royal Navy in the Napoleonic period needs this book. Although it was written as a companion for the Patrick O'Brian's novels (Master and Commander was the recent movie based on the novels), it can be used for better understanding of any of the Royal Navy series (Hornblower, Drinkwater, Kydd, etc.), by anyone who watches the Hornblower movies, or anyone who simply has an interest in maritime studies in the age of sail. The book starts out with sections on the Royal Navy during the Napoleonic Wars, naval medicine, maps, and ship diagrams. You'll learn the names of all the sails and decks on a ship. Then there is a large section, about 350 pages of definitions in dictionary form. It's all here-ship ratings, maps, compass points, parts of ships and cannons, biographies, battles, ranks, and everything else you might encounter as you read the novels or watch the Hornblower movies. If you don't know what a cox'n is, you'll find it here. This book is great not only as a reference, but has given me hours of browsing pleasure. It really opens up a whole new world that this landlubber doesn't understand from first-hand experience. It's a great book--I can't say enough about it. It can be picked up in paperback for pocket change and it's worth its weight in gold. You will be pleased with this book!