In the ranks of summer-vacation stories, Canadian author Katz's American debut fails just short of middling: full of interesting witch lore, much of it peculiar to Guernsey, but burdened by creaky characters and thick plotting. Fourteen-year-old Mike, vacationing with divorced journalist Dad, is the Wisconsin innocent abroad who stumbles on the curious coven-like activities at Trepied tomb. Lisa, his host's daughter, has a theory--her beautiful and resented stepmother is one of the regulars--but Mike is wary until the evidence starts mounting, including a puppy left slaughtered at the scene of a Black Mass. Gradually he sides with Lisa and confides as much to disbelieving Dad. Then he orchestrates a series of happenings to disrupt the evening's proceedings and flush the secret participants out into the open. The struggle involves a powerful book of magic, the Vieux Albert, which is missing when the coven's leader dies suddenly. Before its whereabouts are discovered--it resides in the leader's head, moving after each death--the two members competing for supremacy kill each other off and Lisa's father dies too, unlucky victim of the book's unstable powers. Readers charmed by the witchery detials (a severed hand, symbols on the floor) may nevertheless resist the stock figures and their stiff behaviors. Mike is strictly a golly-gee-whiz kid, Lisa is largely suspicions and pouts, and tiptop journalist Dad misses the story right under his nose. Little magic, then, despite the original touches.