M. V. Carey (1925–1994)

This one had some promising elements, such as a mysterious old potter who goes missing and the convenient arrival of his grown daughter and her teenage son, who is just the right age to throw his lot in with Jupe and the boys to solve the mystery. That's all fine, but the answer to the mystery lies in yet another fictional Eastern European country (this is at least the third, I think, in the first 15 books of the series, no doubt reflecting the Cold War era in which they were written). The biggest irritation, however, was that the boys never actually solve the mystery of the flaming footprints! I mean, they find out who was doing it but the extreme handwaving around the explanation of how they were created was supremely unsatisfying. "It's chemicals!" just isn't the bingo the author seems to think it should be.… (more)