In order for a novel to qualify as a mystery, it must meet these plot requirements:
Cozy mysteries are lighter in tone, don't describe the murder in too much detail, take place in a small town, and the sleuth is an amateur. There are smaller divisions within this subgenre. Hobby mysteries feature a group of characters who all participate in some sort of hobby like knitting, baking, or book clubs. Domestic mysteries feature things around the house, usually a pet dog or cat who aids the sleuth. Some popular authors in this genre are Joanne Fluke, Rita Mae Brown, Laura Childs, Diane Mott Davidson, Cleo Coyle, and Nancy Atherton.
Caper mysteries have a comical element. They feature either a bumbling detective or are told from the point of view of a bumbling criminal and seem more like comedies. This is the only mystery subgenre where the crime isn't always a murder, sometimes it is a heist, kidnapping, etc. Try these authors: Donald E. Westlake, Lawrence Block, Elmore Leonard, Carl Hiaasen
Mysteries in this subgenre always feature a protagonist who is a member of the police force, FBI, or some other official government entity who is investigating the crime. Try these authors: P.D. James, Ruth Rendell, Tony Hillerman, Ian Rankin, John Sanford, Michael Connelly, Jeffery Deaver, James Patterson, Nelson DeMille, C.J. Box, Jussi Adler-Olsen, Jo Nesbo, Martin Cruz Smith, John Lutz, Karin Slaughter, Donna Leon, Ann Cleeves, Patricia Cornwell, Tess Gerritsen, Kathy Reichs
This subgenre is what it sounds like- mysteries with a supernatural element like ghosts or messages from the unknown. It can add another twist to what would otherwise be a regular mystery. Try these authors: Charlaine Harris, Jim Butcher, Victoria Laurie, Madelyn Alt, Juliet Blackwell
Hardboiled and noir mysteries feature professional, independent investigators (often of the trench coat variety) and describe the murders in detail. These investigators are independent and not part of any police force. They are often investigating mob and mafia related crimes that flourished during the Prohibition era (1920-1933), as well as corruption of the police force and legal system. This genre had its heyday in the 1930's-1950's and was popularized by James Cain and Dashiell Hammett. More contemporary novels in this subgenre don't take place during Prohibition, but still feature professional, independent investigators in an urban setting. Some contemporary harboiled authors are Sara Paretsky, Walter Mosley, Sue Grafton, Dennis Lehane, Robert Galbraith, Jonathan Kellerman, and James Lee Burke.
A historical mystery is exactly what it sounds like- mysteries set in the past. Try these authors: Rhys Bowen, Alan Bradley, Agatha Christie, Deanna Raybourn, Susan Elia MacNeal, Tasha Alexander