The Witchmaster's Key by Franklin W. Dixon

Book 55 of The Hardy Boys' Mysteries

There is no time for explanations when Mr. Hardy telephones Frank and Joe from the West Coast and sends them flying off to England to help his old friend Professor Rowbotham. Their stay in East Anglia begins with a weird omen as they witness the bizarre funeral of an old Witchmaster. From then on, strange things happen. The plot deepens when the Hardys learn about Lord Craighead's strange disappearance. Danger follows them to Ireland as Frank and Joe survive a shipwreck and find a clue from a frightened white witch that leads to the torture chamber of a black witches’ coven, all in search of the telltale Witchmaster’s Key.