Piet Potter Strikes Again — Robert M. Quackenbush

Once again, Piet (rhymes with neat) Potter, the amazing boy private eye, is called upon to solve a difficult case right in the building where he lives on New York City's Upper East Side. This time he has to find out who is trying to sabotage the birthday party of one of the high rise's residents. There are three suspects, all of whom have it in for the birthday girl, Rachael, but with Piet's cool sleuthing, the case is brought to a swift close with surprising results.