The Secret of Spiggy Holes
by Enid Blyton (Basil Blackwell, 1940) [reprinted Armada, 1965]
Predating the Barney Mysteries but very much in keeping, distinguished only by the absence of any animal companion, and a return to the Secret Island (from the children’s previous adventure). The cover blurb anticipates reader fretfulness by giving away the entire plot.