The most prominent feature was this was a series of very short kids books where you had to flip the book upside down and backwards to read the answer to the mystery. Only like 5 pages per mystery I think. I think it was also two mysteries per book. One of the stories involved an investigator that had to investigate a death and the butler let him in to the study. The guy was dead at his desk with a bottle of cyanide firmly clasped in his hand. It was determined to be a murder because cyanide causes muscles to relax and the bottle could not be clutched tightly in his hand. It was staged to look like a suicide.