The four stories in Once Upon a Kiss, the newest volume in the Once Upon... series (Once Upon a Rose, etc.), contain elements of the fantastic, but only Nora Roberts and Jill Gregory succeed in making the unbelievable believable. Fate pairs a contemporary P.I. with an Amazonian demon slayer from a parallel universe in Roberts's irresistibly funny ""A World Apart,"" and shades of Harry Potter and Romeo & Juliet can be found in Gregory's ""Impossible,"" a charming fable of two lovers who bring peace to their feuding families through marriage. In contrast, Ruth Ryan Langan (""Sealed with a Kiss"") and Marianne Willman (""Kiss Me, Kate"") stumble with stories that would have been better suited as novels. Overall, this unbalanced collection isn't likely to encourage readers new to the series to look up the previous books, but there's no question that it will grace bestseller lists.