Gallows Thief by Bernard Cornwell

Gallows Thief (2001) is a mystery novel by Bernard Cornwell set in London in the year 1817, which uses capital punishment as its backdrop. Rider Sandman, a veteran of the Napoleonic Wars, is hired as an investigator as a formality to rubber-stamp the death sentence of a condemned murderer. Instead, he discovers a conspiracy to conceal the real killer. In the slang of the time, a “gallows thief,” (also a “crap prig”) is a person who prevents the hanging of an innocent person.