The Secret of Moon Castle
by Enid Blyton (Basil Blackwell, 1953) [Armada, 1991]
Blyton’s ‘Secret’ series ends with a bit of a whimper. The villain is unambiguous and the mystery entails a nebulous mining scheme that, Scooby-Doo style, would have remained hidden if not for his trying to scare the kids off with manufactured spookery.