The Hired Man - A Dick Hardesty Mystery, #4

When private investigator Dick Hardesty is hired by businessman Stuart Anderson to conduct routine background checks on potential store managers, he becomes reacquainted with a former trick, Phil Stark, who has undergone an amazing transformation from bar hustler to professional escort. When Anderson is murdered, Hardesty is hired by the escort services owners, Arnold and Iris Glick, to keep Phil and the agency away from police scrutiny.
Two subsequent murders make this impossible, and Hardesty embarks on a mission to find the identity of the killer.