Shipwreck on the Pirate Island is the 18th book in the series.
Geronimo goes to the pirate islands in search of treasure.
My sister had come up with a new way to torment me. She'd combined my two least favorite things: travel and ghosts! Thea had heard rumors of a haunted pirate treasure buried on a desert island. And before I could say "Avast ye scurvy rats," she'd dragged me into her treasure hunt!