THE FASHION CYCLE by Irene Daria (Simon & Schuster: $21.95; 240 pp.) Perhaps it's only proper that a book about the fashion industry should seem better on the hanger than it does when you get it home and try it on. Irene Daria came up with a great idea: Follow five designers, from couture's Arnold Scaasi to volume sportswear's Liz Claiborne, through a hectic year for a behind-the-scenes look at the rag trade. And she had decent access, even if Donna Karan did put her on hold for six months. But the book is less than the sum of its parts. God may be in the details, but the merely mortal reader requires a framework, a depth of perception that the author fails to provide. Her story lacks the writerly touches; a sense of the personalities she met is almost completely lacking save for the reporter's standard description of dress and physical attributes. The story brightens only by coincidence, when the facts are just too funny to be ignored--like the scene among Karan, her stepdaughter and daughter as they consider the designer's new lower-priced line.