The thing about a Mark McShane mystery novel is that he never repeats himself; he's endlessly inventive and his books are entertaining. In 1963, there was Untimely Ripped, a puzzler in the Jack-the-Ripper vein. Last year there was Night's Evil, which successfully projected the ambience of English carney life and an aging detective. Now, the sure comedy of Little Doom, a Cornish catastrophe involving the fifteen village inhabitants, is a near-takeoff of the Christie classic And Then there were None. A blue ribbon crimson madness.