The title says it all--along with the subtitle: ""A sexy fairy tale."" Yes, like last year's Miss Nobody by Caroline Ross, this is an imaginary affair between the Prince himself (called James here, but obviously Charles in all respects) and a commoner. But, while Ross played the gimmick for lacy sighs, Spiegel plays it--at undue length--for titillation and low humor. . . as well as icky sentimentality. Millie, daughter of immigrant-Jewish Bronx-dwellers, is a freelance ""script supervisor"" for N.Y. commercials. . . when utterly perfect Prince James (still unwed) comes into her studio to tape a Magna Carta talk for public television. They make eyes at each other. She spills coffee on him. He drops in at her apartment. She resists his sex-on-the-first-date approach, but soon they're going at it avidly. And, when not in bed, he's telling her about Buckingham Palace while she tells him about bagels and lox. So it goes--with the inevitable culture clashes (a weekend with Anglo gentry in Newport), the ducking of reporters, the spats (an idiotic situation-comedy misunderstanding). . . and the serious discussion about marriage possibilities. (Morganatic? Abdication? Conversion?) But finally, of course, James must do his duty, and Millie will have her work. . . as well as a little something (someone) to remember the Prince by. A serviceable short-story idea padded out to novel length with weak N.Y. comedy, graphic prince-sex, okay film-studio detail, and unconvincing English-accent dialogue: possible diversion for those who usually get their fiction only from Cosmopolitan, Playgirl, or the mass-market paperback rack.