The real mystery about this mystery-comedy-romance is how it ever got to be a Book-of-the-Month Club selection. Admittedly, first-novelist Isaacs and her housewife-detective-narrator, Judith Singer, come across with easy wit and likable smartsiness when introducing Judith's Long Island upper-middle-suburbs milieu--the milieu also of Casanova dentist Bruce Fleckstein, whose mysterious murder has Judith all keyed up. And, as curious Judith starts digging up more and more about Dr. Bruce's kinky conquests (including some of Judith's best friends!), we're ready to breeze happily through another blood-and-fluffer, pleasantly forgettable even if the mystery itself happens to be a third-rate puzzle. Isaacs, however, has other ideas. She wants us to take Judith's amateur sleuthing seriously and realistically, to accept it as her way of fulfilling herself (she abandoned a Ph.D. to have babies), her way to break out of a dull marriage, especially when the sexy cop on the case starts stirring up her juices. Unfortunately, this welding of the formula mystery-comedy with the self-realization novel is so unconvincing that we end up siding with pompously disapproving husband Bob, who seems to be in one book while Judith dabbles in another, fulfilling herself by wearing her bullet-proof vest as bait in a trap for the killer. The stuffy husband bit and the female lust bit (""I dream of finding some nice, seedy motel room and screwing until we both expire from exhaustion"") have been done better--and to death--by the Sue Kaufmans and Erica Jongs. And better mysteries are a dime a dozen. In fact, the only things that are Isaacs' own here are that sweetly satiric tone and a sure ear for contemporary dialogue; we look forward to hearing those sassy sounds in far better, less clumsily contrived, books ahead.