The Mystery of the Disappearing Cat
by Enid Blyton (Methuen, 1944)
In this second book, Blyton still has the Five Find-Outers (and dog) as a fairly generic group of kids happening upon a mystery and solving it largely through happenstance. A spot of Goon-baiting aside, they are yet to come into their own.
The Mystery of the Secret Room
by Enid Blyton (Methuen, 1945); audiobook read by Ann Beach (Chivers, 2010)
The third book in the series but perhaps the best place to start if new to children’s detective fiction and/or the Five Found-Outers (and dog). Fatty and the others come into their own, outwitting Goon the policeman while solving another mystery.