The 1920s were a decade of change, innovation, and experimentation in the mystery genre. It was during this decade that the genre began to take on a more realistic and grounded approach, with an emphasis on police procedural and detective work. Here are some notable proto-police procedurals and police procedural mystery books from the 1920s:

The Benson Murder Case by S. S. Van Dine (1926):

The Benson Murder Case is the first in a series of detective novels featuring Philo Vance, an aristocratic amateur detective who uses his intelligence and deductive skills to solve crimes. The novel is notable for its meticulous attention to detail and its emphasis on forensic evidence, making it one of the earliest examples of the police procedural sub-genre.

The Cask by Freeman Wills Crofts (1920):

The Cask is a classic example of the “inverted detective story” sub-genre, in which the identity of the killer is revealed at the beginning of the novel and the focus is on how the detective solves the case. The novel features Inspector French, a meticulous and methodical detective who uses careful observation and scientific analysis to solve a seemingly unsolvable murder case.

Trent’s Last Case by E. C. Bentley (1920):

Trent’s Last Case is a groundbreaking novel that marked a departure from the traditional “whodunit” mystery format. The novel features Philip Trent, a freelance journalist who is hired to investigate the death of a wealthy businessman. The novel is notable for its emphasis on character development and psychological depth, as well as its use of multiple narrators and unreliable witnesses.

The Roman Hat Mystery by Ellery Queen (1929):

The Roman Hat Mystery is the first in a series of detective novels featuring Ellery Queen, a detective who uses logic and deduction to solve crimes. The novel is notable for its attention to detail and its use of scientific analysis to solve the murder case.

The Man in the Brown Suit by Agatha Christie (1924):

The Man in the Brown Suit is a classic mystery novel that features Colonel Race, a police officer who uses his expertise and experience to solve a murder case. The novel is notable for its fast-paced action and its use of international settings and characters.

In conclusion, the 1920s were a decade of innovation and experimentation in the mystery genre, with an emphasis on police procedural and detective work. These five notable books showcase the emergence of a more realistic and grounded approach to mystery writing, which has had a lasting impact on the genre.