The Green Jade Hand, by Harry Stephen Keeler (epub/Kindle/pdf)

“Exhibition Extraordinary!” So began the poster advertising the professional debut of Simon Grundt, formerly of the Lincoln School for the Feeble-minded. How the police could choose Simon to solve the murder of Casimer Jech, rare bookdealer, is a tale only Harry Stephen Keeler could have chronicled. Before it’s done you’ll meet ex-con Luke McCracken, ’bo Tom Steever, landlady Sadie Hippolyte, inventor Dirk Mattox and his fiancée Iolanthe Silverthorne, and wealthy gad-about-town Oliver Oliver. Not to mention a host of celestials and a gaggle of cops—all of them affected one way or another by the six-fingered green jade hand.

A note for the politically correct: this book is decided not. Keep in mind the year in which it was written (1930). It reflects the times.