Larry Dexter - The Stolen Boy

Book 5 in the series: Fresh from success in solving a big crime that not even the police could get a grip on, Larry is sitting at a recital for an opera star about to return to Europe when he sees a strange man making signals, and the singer suddenly faints. It turns out that her has a son who has just been kidnapped, and recognized the man as someone who had threatened her before. Larry's search takes him around the New York area and then off on a train to the Great Lakes where he teams up with a millionaire friend who has just the right boat to set out after the kidnappers. Action, adventure and even a fight ensue.