Cassie Sales is 50, dumpy and bored, and she's the rather spiritless heroine of this new novel by Glass (Burning Time). Cassie's jet-setting international wine-dealer husband, Mitch, hasn't had sex with her in years; to reignite their failing marriage, Cassie resolves to take advantage of Mitch's business-trip absence and buy herself a face-lift. She has the surgery, but Mitch arrives home unexpectedly, takes one look at her frighteningly bruised face and collapses with a near-fatal stroke. He lies in a coma as Cassie slowly learns that he's been cheating on her for years and is about to end their marriage and leave her broke. What should be a comic romp starts off as a plod, weighed down by the musings of the earnest and pathetic Cassie. The novel picks up halfway through, when the point of view shifts to that of Mona, Mitch's manipulative 36-year-old mistress. She's a shallow but vigorous conniver whom Glass describes with obvious relish ("Mona was a very practical girl whose bible was The Art of War.... She analyzed it daily and applied the strategy of the Seven Military Classics to human relations"). Glass's portrait of Mitch, "a very dependent man parading as an independent one," is also sharp and believable. But secondary characters, such as Cassie's slapstick Aunt Edith or her bratty children, Marsha and Teddy, are less successful. There are bright spots, but readers of Glass's far better mystery series featuring NYPD detective April Woo should skip this one. (Mar.)