Piet Potter Returns — Robert M. Quackenbush

Piet (rhymes with neat) Potter, the amazing boy private eye, learns that someone has been tampering with the elevators in his building on New York's Upper East Side. It could be the same person who has been causing mischief in the neighborhood park. With his new friend Amy, who writes a newspaper for the tenants of the building, Piet sets to work on the case and uncovers some surprising evidence.