The nature of the self, and how it mutates, is a recurring theme in the four novellas making up London-born Stevenson's smart debut. In ""The Island of the Day Before Yesterday,"" pretentious Italian semiotician Simone Strachey is summoned by the Sunday Times to organize the estate of his dead father, a minor member of Rome's literati. The humorously erudite protagonist goes to London and hires a frumpy secretary to help sort through the papers, but he soon gets his comeuppance amid Stevenson's delightfully nimble turns of phrase. When Simone, misguidedly altruistic, decides to pretend that ""Dreary Dora"" was a member of the late Strachey's glamorous scene, she winds up posing as Simone's effervescent, underrated wife, a '6os cult figure, and steals the show. ""Law and Order"" is a dark glimpse into the unraveling relationship of twins Henrik and Florian, who, while studying law at the University of Leiden, fall under the spell of a professor who has frightening views about crime and responsibility. ""The Colonel and Judy O'Grady"" chronicles a grad student's infatuation with exotic Ananda, formerly Judy O'Grady and currently a Buddhist nun who has erased her past. The creepy ""Crossing the Water"" is a sinister finale, with unemployed artist Oliver on a lake trip with a bunch of upper-crust art historians whose class snobbishness may play a part in their incipient tragedy. Most of the book's characters are highbrow Europeans, and their diction may be off-putting to some readers. But Stevenson realizes the hilarious parodic effect of their ultra-proper intonations, especially when she places them in deliciously vulnerable situations. (Sept.) FYI: The author's upcoming novel, London Bridges, will be published by Houghton Mifflin in 2001.