Before the famous The Floating Admiral (1932), Christie, Dorothy Sayers, and their London Detection Club chums collaborated on two novella-length serials--here published in book form for the first time. The Scoop is by far the better of the two, blending newspaper-camaraderie charm into a solid enough little puzzle: the murder of a young woman in a Lonely Bungalow, with her disappearing "husband" and her lover as prime suspects. . . and with a romantic/comic news-team as sleuths. (The Christie and Sayers chapters are noticeably brighter than those by such other contributors as Freeman Wills Crofts and Clemence Dane.) Behind the Screen is more of a mere stunt: the murder of a boarder in a large household; motives for everybody; and dozens of loose ends in the early chapters that had to be resolved, less than brilliantly, by the writers--E. C. Bentley, Ronald Knox--assigned to the convoluted windup. ("Robert knows that the second blow was the fatal one, therefore he pretends that he dealt the first. Miss Pettigrew thinks that the first blow was fatal; therefore she pretends that she dealt the second," etc., etc.) More nostalgia than topnotch mystery, then, but the Christie touch is definitely, agreeably in evidence--especially in The Scoop.