The Castle of Adventure
Enid Blyton
 Jack, Lucy, Dinah and Phillip attempt to figure out what is behind the strange goings-on at a ruined castle near Spring Cottage in Scotland where they are on holiday with Dinah and Phillip's mother, Aunt Allie. The four youngsters make friends with Tassie, a mysterious gypsy living in the woods with her mother. Along with Bill Cunningham, who appears later in the book, the children manage to expose a ring of spies led by the threatening Scar-Neck who are working against the UK Military service.