Mystery of the Martello Tower — Jennifer Lanthier

Hazel and Ned are home for summer vacation and looking forward to long, lazy days of sleeping late, shooting hoops, building stink bombs, and spending time with their art-dealer father. But when he disappears without saying good-bye, their summer plunges into chaos. The babysitter leaves town, their apartment is burgled, and two menacing thugs start turning up everywhere. Ned and Hazel try escaping to an island castle belonging to long-lost relatives, only to find there's no escaping this adventure. As the siblings work to untangle the threads that ensnare their father, they learn of a second, darker secret -- one that surrounds their mother's death years ago. Only by solving both mysteries can they bring their father home.