To commemorate its 50th year in business, Signet is issuing its first hardcover book, for which it commissioned 18 new stories from some of the most popular authors published by the house, including Stephen King, Joyce Carol Oates and Ed McBain. Beginning with Lisa Alther's ""Stormy Weather,"" in which an ex-hippy happens to eavesdrop on a conversation between two of her ex-lovers on her 50th birthday, several of the collection's tales are concerned, predictably enough, with what it means to turn 50. These include a cop story from Larry Collins and ""Mother's Day"" by Joy Fielding, whose protagonist is a middle-aged woman who grieves for her mother and feels rejected by her bratty teenage daughter. Some of the pieces seem thrown together, and Stephen Frey's ""Paranoia,"" about a woman who can see the future, is an unremarkable potboiler. On the other hand, Lawrence Block's beautifully controlled story of a psychic who finds murder victims delivers authentic chills. Erica Jong's contribution is the collection's only departure from the short-story form: ""Songs in the Key of I: one essay and a dozen new poems by erica jong."" She makes a cogent plea for the essential presence of poetry in our culture. Though the quality of the contributions varies, the volume nicely showcases the different possibilities offered by short fiction in several genres. (Jan.) FYI: Signet will make a contribution to Literacy Partners in the name of each author featured in The Best of the Best.