This is a short (12 pages of content) and concise guide to creating a mystery that will work in your role playing game. It describes what happens in three acts, and that becomes the plot points in your 3 to 8 adventure sessions. It covers how to add subplots and a dramatic turn that will make the ending more satisfying. The use of NPCs makes it all feel more realistic. It helped me get a handle on what the key NPCs will be doing, what locations I wanted to pre-plan... The prep chapter (3 pages) was helpful. I gave me some handle on what to do if the players guess the villain early -- because knowing who did it is not the same as solving the mystery.
The structure works for any kind of mystery where someone or some organization doesn't want the players to know something. It doesn't have to be a murder.
I found this very helpful, and for a week I took about a half hour on my drive and answered the questions in the book, creating a different mystery every night. Before you know it you can really come up with some amazing stuff.
It could be a little better if it had more specific advice about what to do if players miss a clue, or guess the villain, but there's another here to work with.