Mystery books focus on solving a crime, usually a murder. The crime is the so-called "inciting incident" that sets the events of the book in motion. The protagonist (an investigator), along with the reader, tries to piece together the available clues that lead him or her to the crime's perpetrator. The story usually concludes with the discovery and capture of the evil-doer. The investigator is usually a member of the police force, a private eye, or an amateur detective. The setting may be contemporary or historical.
This blog post elaborates on the difference between mysteries and thrillers.
The mystery subgenres seen here reflect most, but not all, of what's being published today. Book Country's genre map will continue to change as new literary categories emerge and others fall out of fashion. If you write mystery novels, explore a genre to connect with other like-minded writers, workshop, and publish your book.