The Pit-Prop Syndicate is a 1922 thriller novel by Freeman Wills Crofts, one of the leading figures of the Golden Age of Detective Fiction. It was one of several stand-alone novels Crofts wrote following his successful debut The Cask, before creating the character of Inspector French who debuted in Inspector French's Greatest Case (1924).
A boat notionally carrying pit props from the Gironde to the Humber is in fact engaging in illegal smuggling. This leads on to a murder in a London taxi and an investigation by the slow but sure Inspector Willis of Scotland Yard.