Harper, the Paul Newman post-noir picture, is based on 1949s The Moving Target, the first of author Ross Macdonalds 18 Lew Archer detective novels. Screenwriter William Goldman updated the story to then-present day (1966) Los Angeles.
Harper Recalls Noir Classic The Big Sleep
According to TCM.com, the films (and title characters) name change came at Newmans mildly superstitious request. The actor enjoyed big 60s hits with The Hustler and Hud, and hoped another H title would make a nice trifecta.
In many ways, Harper echoes vintage films noir, particularly 1946s The Big Sleep. Makes you wonder how much more Harper might have resembled Sleep had Goldman retained the novels