Smith, apparently unaware that there already exists a mystery-series (by Max Allan Collins) featuring a private-eye named Mallory, here introduces James Maxfield Mallory, Boston shamus--in a routinely plotted but agreeably delivered fiction-debut. Mallow, a Harvard grad and law-school dropout with an office adjoining a Combat Zone gym, is hired by cool society matron Morgan Streeter to investigate comely young lawyer Susan Winston--the latest amour of Mrs. Streeter's randy stepfather, Caleb Johnson. (Is Susan just a gold-digger? Can she be bought off to avoid family scandal'?) But Mallory's sleuthing begins too late: by the time he seeks Caleb out at a posh country club, the May-November couple is already married. Furthermore, Caleb is soon dead--murdered via a poisoned bottle of $2000 scotch. And, despite the usual brushes with menacing, faceless assailants, Mallory digs into the possible murder-motive sources: Caleb's gambling debts; family sex-secrets centering on the tree parentage of Mrs. Streeter's immature daughter Melissa; and questions about the management (by an upper-crust law firm) of the Streeter clan's huge trust funds. The unraveling, despite a decorative showdown at the Gardner Museum, is thoroughly ho-hum; the characters--including Mallory, his workaholic girlfriend, and the semi-stereotyped suspects--remain sketchy, only half-intriguing. Still, there's modest promise here--in the crisp Boston-area backgrounds (including a few nicely offbeat vignettes) and in the unforced, gently ironic narration.