In 1975 I read a young adult novel about a city boy sent to stay the summer at his grandmother’s house in a small town. He doesn’t want to be there and expects to have a boring time, but stumbles across a mystery that he decides to solve. None of the adults in town believe him, but he eventually solves the mystery with the help of an older, wisecracking boy from the town. I don’t remember any details other than that the town was very small and along a small river or large creek and his grandmother’s house was at the edge of town near the river. I think the story was set in the generic “midwest” or “northeast”. The book was probably written in the mid- to late 1960s or early 1970s. I don’t think it was part of a series.