"The American Agatha Christie," as she is sometimes called, Mignon G. Eberhart has a huge following among mystery buffs. Her adroit style and penchant for chilling atmosphere are evident in "The Patient in Room 18," her literary debut of 1929. It introduces the emphatic Nurse Sarah Keate, who helped popularize mystery novels and movies set in hospital wards amid the ominous gleam of medical instruments. Eberhart once said of the redoubtable, red-haired Nurse Keate, "I loved her because she had a good sharp tongue." The head nurse needs all her wits in "The Patient in Room 18," which begins off-duty with an unpleasant dinner party and mixes radium with murder, drawing in the cunning Detective O'Leary, beautiful Maida with the lapis lazuli cufflinks, and sinister Corole.