This book, The Technique of the Mystery Story, by Carolyn Wells, was written in 1913 and is now in the public domain.
Chapter 1: The Eternal Curious
Chapter 2: The Literature of Mystery
Chapter 3: The History of Mystery
Chapter 10: The Rationale of Ratiocintation
Chapter 12: Other Detectives of Fiction
Chapter 15: Footprints and Fingerprints
Chapter 19: Persons in the Story
[...] I’ve put up an ebook on writing (murder) mystery stories. It’s called The Technique of the Mystery Story. Written by Carolyn Wells in 1913. Gives advice for writers — what to do, what not [...]