BOOK REVIEW : A Fascinating Story of Addictions : Me and You by Margaret Diehl (Soho Press: $18.95; 258 pages).

Enter Gwen, a 29-year-old alcoholic living on her trust fund in Berkeley, spending her days reading horror novels and watching “Sesame Street,” her nights with her “Cro-Magnon hulk” boyfriend, Lenny. As matters fall out, Lenny is merely pro tem, gone by Page 22, when Gwen is hauled off to New York by her loving sister Lucy, to try, one more time, to sober up and put her life together.

No one said “back together,” because Gwen has been a lush since puberty. She began drinking to keep her adored father company, and then, after her parents’ divorce and her dad’s premature death, she drank to remind herself of their secret revels and to perpetuate his memory. That’s all she’s done with her life since dropping out of a series of first-rate schools, dissipating her talent as a painter and alienating herself from her despairing family.

Miraculously, her looks haven’t suffered, though her liver is wrecked. Delusive, vain, selfish and supremely articulate, Gwen is a heroine calculated to tax a novelist’s powers to the utmost.

Diehl reveals Gwen’s convoluted mental processes with a skill that’s lyrical and clinical, allowing her to tell her own story in the first person. We hear about Gwen and Lucy’s privileged girlhood, an idyll during which Lucy was every bit as wild as her sister. For Lucy, that was just a temporary phase, and she’s now a successful journalist about to marry a thoroughly appropriate young man. This visit to Berkeley is a rescue mission.

Gwen is next seen several months later, standing in the garden at Lucy’s wedding, sipping her designer water and lime; miserable and rebellious. She goes to AA meetings and has found a temporary vocation painting dog portraits for a society clientele.

“I didn’t cause trouble anymore. . . . I was polite to my mother’s friends, whose favorite question was when I was getting married, asked in a sly and joking tone, as if Lucy had pulled off a coup. . . . I said I’d been married secretly but my husband was a freak and wouldn’t come out in public.”

Sober and semi-rehabilitated, she is maid of honor, playing opposite the bridegroom’s father, the best man. The ceremony is barely over before Gwen and Jack become the Me and You of the title. The fact that Jack is married and old enough to be her father simply intensifies Gwen’s interest. Isn’t he exactly what she’s been missing? He’s even a writer, like the father she lost. “I knew what I wanted and it wasn’t an in-law.”

Within three days, they’re lovers. Ironically and conveniently, Jack’s wife, Ruth, has gone off to the Soviet Union on a fact-finding mission, a device that efficiently removes her from Manhattan for six weeks, guaranteeing that any facts she finds will be irrelevant to her life. In that time, Gwen becomes as addicted to Jack as she was to alcohol, demonstrating an eroticism as uncontrollable as her previous attachment to drink. “How this suited my addict soul: the pleasure round about, unchanging askew.”

Through torrid afternoons and passionate nights with Jack, she learns to understand herself, though that self is never transformed. Gwen is a destroyer, and her salvation through love is bought at the cost of Jack’s marriage, and almost of his sanity.

For any man of 50, “Me and You” should be read as a cautionary tale. For the rest of us, it’s a fascinating study of a pathological personality told by a writer at ease with the enormous challenge she’s set for herself.