Lately it seems to be taking over the non-Wexford novels, and, like P. The Rendall hallmark is exposing the hidden guilt in every suspect, and this psychological slant is evident in all her mystery novels to some degree. The Wexford characters are allowed to change and grow while enriching, not damaging, their little world.

Miss Rendall portrays the little world and its inhabitants shrewdly and adeptly and constructs complicated, twisting plots. The Wexford novels are among the best in the mystery genre. Ruth Rendall has written more than 20 novels, 12 of them constituting her Chief Inspector Wexford series. It is gruesome and macabre in its subject matter, tinged with the themes of ''Innocent Blood.'' Cordelia Gray, that heroine of the small world of murder, seems out of place in the distasteful plot. This novel is different from the earlier mysteries, however. In her most recent novel, ''The Skull Beneath the Skin,'' she returned to the mystery format and to her young sleuth, Cordelia Gray. In ''Innocent Blood'' she abandoned the mystery format and its little world and wrote a psychological novel about incest, revenge, and madness. James sets up the little world of Detective Chief-Inspector Adam Dalgliesh of Scotland Yard and Cordelia Gray, girl private detective, in seven excellent mystery novels. And the sleuths - the policemen, the private detectives, and the amateurs - are themselves ordinary people. This small world comprises little English towns and villages and ordinary people from all walks of life whose lives are placed under the microscope of investigation when someone they know is murdered. Ferrars has called ''The Small World of Murder ,'' the title of one of her novels. James, both of whom have been compared to Agatha Christie, have been moving away from what their fellow mystery writer E. The cozy little world of the classic British mystery novel is breaking down and may eventually collapse altogether.

Thinner Than Water by E.X. Ferrars