An ambitious project . . . a pastry-rich and knowledgeable portrait of Paris, with details that redolently evoke but never overwhelm, and Khayyam is an interesting and cosmopolitan protagonist (she’s French, American, and Muslim and comments thoughtfully on the contradictions and intersections of those cultures) with an unusual vocation. Altogether, it’s a Parisian romance with a seasoning of intrigue, a dollop of feminism, a soupçon of caper action, and a lashing of scholarship.