for Captain Jake by Shannon Stewart and Ben Hodson
Jake loves to hunt for treasure, so when the famous pirate Captain Kidd asks him to be his cabin boy, he can't refuse. But Jake soon learns that bringing home an invisible pirate can be a real disaster, particularly when the pirate is mortally terrified of his teenage sister. There are many rules of the sea, and Captain Kidd's own cabin boy, Richard Barleycorn, teaches Jake how to face his biggest fear, Boris Baxter, the meanest boy in the whole school.
Publisher description retrieved from Google Books.