Houseboat Mystery (The Boxcar Children, #12)

Henry, Jessie, Violet, and Benny used to live alone in a boxcar. Now they have a home with their grandfather and are spending a vacation traveling down the river on a houseboat.

Throughout their journey, the Boxcar Children have the feeling they're being watched. Little do they know, someone is looking for something on their houseboat - and he'll stop at nothing until he gets it.