Rex Zero and the End of the World by Tim Wynne-Jones

The year is 1962, and almost-eleven-year-old Rex and his family move to Ottowa, where he joins forces with a band of local kids trying to convince the authorities that an escaped panther is on the loose in their town. It's cute, and seems to capture the period well, but felt too young. NAY, though I'd definitely recommend it to my ten-year-old nephew!