Books like The Adventures of the Black Hand Gang

God, I loved this SO MUCH. The book comprises four stand-alone mystery stories. The reader has to locate the clues hidden in the charming illustrations ¹ to solve each step of the investigation. I think I was entranced less by the mysteries and more by the four kids roaming around a busy town by themselves all day.¹ Although I guess you could just turn the page and find out what you were supposed to have seen? 8-year old me would never have done that.