EVERYBODY LOVES GETTING something for nothing (i.e., "free"—but take our word for it, my son, nothing is ever really free). Nevertheless, we've come across several "free" texts relating to crime-mystery-detective fiction that you might find useful as references:
(1) The Cambridge Companion to Crime Fiction (2003) (PDF).
Edited by Martin Priestman. Online (HERE).
(2) Crime Fiction—The New Critical Idiom (2005) (PDF).
By John Scaggs. Online (HERE).
(3) A Companion to Crime Fiction (2010) (PDF).
Edited by Charles J. Rzepka and Lee Horsley. Online (HERE).
There's no telling how long they'll be there, so you'd better get a move on.
The bottom line: "Thought is free."