He wrote Long John Silver, The true and eventful History of my Life of Liberty and Adventure as a Gentleman of Fortune and Enemy to Mankind. This is his supplement to Robert Louis Stevenson's Treasure Island, the memoirs of Barbecue as told by Long John Silver himself.
An ordinary person who has never read Treasure Island and knows nothing about historical piracy can certainly enjoy this book. It's clever, with subtly recurring themes and rewarding characters. There is plenty of action as well as philosophy, and the reader would no doubt be left with a much better understanding of the machinations behind The Golden Age of Piracy than they had upon opening the book to page one.
A person who has read Treasure Island yet knows nothing of historical piracy would enjoy the book even more. Larsson fleshes out the characters the reader already met in Stevenson's masterpiece, giving them the depth, quirks, and backgrounds that make people more interesting than cardboard cutouts. The reader would benefit from the viewpoint of an older, mature man rather than a young boy, and revel in the "private joke" feeling one gets from surreptitious mentions of the other work.
A person who has read Treasure Island and has knowledge of historically accurate piracy will have an intellectual orgasm. Larsson, a seasoned seaman himself, has taken a fictional character and literally inserted him into history. He manipulated Silver's life to fit among the lives of real pirates of the era. Silver speaks with Defoe in a cafe, and sails as quartermaster on England's ship. He spends time on slave ships, with the boucaniers, and rejoices with the crews of various vessels on Johanna and Sainte Marie.
Not only has Larsson done his homework on the reality of the lives of pirates (including a brief and subtle allusion to sodomy in the second chapter! haha), he stepped into Silver's skin with admirable agility. The narrator is not a Swedish author trying to keep his keyboard steady while his sailboat rocks in the North Sea--you are hearing Long John Silver's own words and thoughts. He's cynical, he hates God, and he cares about nothing but the prolonging of his own life. Through this sarcastic, self-serving gentleman of fortune, Larsson portrays the piratical life the way it really was: full of lust and thirst and aggression. His characters are not seeking treasure, they're seeking life and death "in advance," in his own words.
If you're a pirate enthusiast, you simply have to read this book, and it wouldn't hurt you to read it if you're not. In fact, you may well become one afterward.