Balancing Acts : LILY WHITE.<i> By Susan Isaacs (HarperCollins: $25, 480 pp.)</i>

A one-volume vacation reader, “Lily White” combines the allure of a legal mystery, a romance and a dysfunctional family saga in a single portable package. In fact, the reader gets two of everything--a convoluted capital case, plus a shorter, bonus one for good measure. The heroine enjoys a pair of great passions and there are even matched sets of uncaring parents, Lily White’s and Jasper Taylor’s, each couple failing their children in their own way. Lily’s difficult sister and Jasper’s problem brother add additional weight.

Keeping all these elements in balance requires the skill of an acrobat and, for the most part, the author rises to the challenge, helped by her sense of irony. The plot moves briskly along, but often at the expense of the characters. Lily’s fashion-obsessed mother and her remote, success-driven father are familiar models of upward mobility, while Jasper’s parents are mere aggregates of dated upper-class attitudes: all stock types from the 20th century fiction warehouse.

Because exposing the eccentricities of her husband’s family and those of her own requires the narrator to depend heavily on flashbacks, we’re frequently whisked out of the courtroom where the adult Lily is defending an accused murderer and taken to Shorehaven, Long Island, where Lily and Jasper are teenagers. These interludes, though necessary to the family chronicle, have an unfortunate tendency to diffuse the impact of the legal story.

After growing up in different but contiguous worlds, Lily and Jasper meet in law school, fall in love and marry. Life is grand until Jasper (renamed Jazz) gets bored with shuffling corporate assets from one bank to another while his bride (now a more sophisticated Lee) works in the Manhattan district attorney’s office, prosecuting malefactors like Fat Mikey Lo Triglio, “a vulture feasting on society’s entrails.”

Mobilizing all his charm, Jazz persuades Lee to leave the city and the job she loves for a return to Shorehaven, where he plays country squire and she starts defending the same sort of people for whom she had been seeking 20 years to life.

Lee is retained by Norman Torkelson, a con man indicted in the murder of his latest mark, the spinster Bobette Frisch. Norman has a well-documented habit of seducing lonely women. Pleading urgent financial difficulties, he accepts sizable “loans” from his victims, then vanishes to resurface in another city, with a new batch of credentials and a fresh list of possibilities. Because the women he swindles are ashamed to report him, he has eluded capture until Bobette is found strangled and an eyewitness puts him at the scene. Lee eagerly accepts his case.

Norman might actually be an innocent man wrongly accused. Although by no means a model citizen, he has hitherto been content to take the money and run, stoutly insisting that he has never physically harmed any of his women. Lee wants to believe him.

At this point in her life, she needs to believe in something, because her domestic life has deteriorated since her return to Long Island. Utterly disenchanted with law, Jazz has gone into business with Lee’s father, an upscale Manhattan furrier. While Jazz enjoys his mid-life career change and the prosperity it brings him, Lee is not nearly so glad to relive the life she hoped to escape by marrying into the socially prominent Taylors of Hart’s Hill and becoming an attorney. (Teasing allusions to a mysterious “guy” have hinted all along that this person may well not be Jazz.)

Lee throws herself into Norman’s defense with extraordinary energy. Her involvement gives the book form and direction for as long as she is able to keep her client out of jail, but once Norman is found guilty, the multiple story lines begin to unravel. The second, less interesting case is offered up as a consolation prize, adding further complications to an already convoluted plot. Lee’s sister turns up with a substance abuse problem, sending the story line careening off into soap opera land.

Eventually the identity of Lee’s mysterious guy is revealed, though by then, the surprise is diminished.

After several more sharp turns and unnerving skids, the saga of Norman is finally resolved, long after it seemed concluded. Justice is served and love triumphs.

“Lily White” ends optimistically and somewhat unconventionally but, according to the narrator, “As it turned out, the way it often does with choices made with wide-open eyes and wise hearts, it was fine.” To disagree seems churlish, though the route has been somewhat more circuitous than necessary.

*

“Lily White” is also available on two audiocassettes from Harper Audio for $18.