Oldies but goodies:
Encylopedia Brown (one chapter mysteries with the answer at the back of the book)
The Henry Reed series about an entrepreneurial young boy who starts a lot of businesses, written in an epistolary format with illustrations. There is a slightly quirky angle to the stories where things often go wrong in funny ways before he and his friend Midge find a solution.
https://en.wikipedia.org/wiki/Henry_Reed_(character)