Look at this thing: 1920s guide to becoming a private detective
"Oy, those elbows got the shovers of the queer in the sweat box!"
Introducing the Secret Intelligence Service manual:
If you’ve ever fantasized about being a private investigator — specifically from the era when people talked about stool-pigeons and coppers — then this is the book for you. Well, “books,” technically. This is a set of 21 pamphlets which, together, make up a complete course in becoming a “secret intellig…