Jacquelyn Mitchard, . . HarperCollins, $25.95 (400pp) ISBN 978-0-06-058724-6

No one could blame Julieanne Gillis, beleaguered heroine of this no-holds-barred family drama by Mitchard (The Deep End of the Ocean , etc.) for not seeing the signs. At first her lawyer husband, Leo Steiner, seems to be in the throes of a midlife crisis, informing Julieanne that he is planning to take early retirement and go and live on a commune in upstate New York for six months. The next thing she knows, he's vanished, leaving her with three children and only her meager income from her advice column for the Sheboygan, Wis., local newspaper. To make matters worse, she's diagnosed with multiple sclerosis. The narration alternates between plucky Julieanne and her 15-year-old son, Gabe, a handsome Holden Caulfieldesque loner with a mild learning disability. When things get desperate, Gabe and his 14-year-old sister, Caroline, scan their dad's old e-mails and learn where he might be. Then, during spring break, lying like troopers, the two juveniles take off by bus to find their father. Surely, they think, he'll come home when he learns that their mother is sick. He comes, but the baggage he brings along means further disaster. Leo's behavior is almost campishly craven, but the novel's soap-operatic bathos is perversely satisfying. Rousing melodrama; fluid, often funny, dialogue; and the convincing portrayal of children involved in the collapse of a marriage add up to another page-turner from Mitchard. Agent, Jane Gelfman. (Apr.)