The Devil Wore Prada has come to serve, much as Bridget Jones' Diary used to do, as the title conveying the essence of a certain kind of book, usually a form of chick lit involving young career women on the make. Now Maureen O'Brien, executive editor at HarperEntertainment, thinks she has the next hit in the genre, taking it to Hollywood. It's called Emily's Reasons Why Not and in it first-time novelist Carrie Gerlach, a movie publicist and would-be screenwriter, writes of a heroine, much like herself, seeking the right man among the innumerable wrong ones available in the movie capital. O'Brien enterprisingly flew to the Coast before BEA, picked up the manuscript from Alan Nevins at The Firm, read it on the plane home and preempted for world English as soon as she landed. She looks to publish, possibly at Morrow, next spring.