Lily and Dash: No Nick and Nora : HELLMAN AND HAMMETT: The Legendary Passion of Lily and Dash.<i> By Joan Mellen (HarperCollins: $32, 549 pp.)</i>

A few years ago, working on a piece about the school of hard-boiled detective fiction, I reread Dashiell Hammett’s “The Thin Man” for the first time in 15 years and was shocked. The wise-cracking private eyes Nick and Nora Charles of my memory had been replaced by a pair of cynical, hard-drinking shrews who seemed at least as unsavory as the characters they were trying to put in jail.

The reason for my faulty memory is simple: The Nick and Nora I liked were created by William Powell and Myrna Loy on screen, while, as Joan Mellen reveals in her meticulously researched new dual biography, “Hellman and Hammett,” the Nick and Nora of the novel were modeled after the real Dash and Lily.

The three-decade relationship between Dashiell Hammett and Lillian Hellman ranks with Scott and Zelda and Jean Paul Sartre and Simone De Beauvoir in 20th century literary legend. But while the others merited individual biographies or studies, Joan Mellen makes it clear that the stories of Dash and Lily can’t be separated: Hammett’s career as a writer was nearly over by the time he met Hellman in 1930, which is where the literary portion of Hellman’s career and the interesting part of Hammett’s life begin.

“Hellman and Hammett” is skimpy on the early part of Hammett’s life and the events that led up to the writing of his classics, “Red Harvest,” “The Maltese Falcon” and “The Glass Key.” For those subjects one is advised to check out, respectively, Diane Johnson’s “Dashiell Hammett: A Life” and William F. Nolan’s “Dashiell Hammett: A Casebook.”

But Mellen’s book is definitive if only because it gives us an enormous amount of information on what we haven’t been told about Hammett before, largely because Hellman succeeded in keeping it from us.

For instance, Mellen proves beyond doubt that Hammett was indeed a Communist, or at least a fiercely loyal supporter of the American Communist Party. (His introduction to communism was through the Baltimore longshoremen of his youth.) Hammett did father a daughter, Josephine, by Jose Dolan, a nurse he met while in a Tacoma, Wash., hospital, but Mary Jane, long thought to be his oldest daughter, died without knowing that Dash wasn’t her natural father. And Josephine had to wait 30 years to receive her inheritance; Hellman appropriated the money.

The last bit of information is merely one of the nasty shockers Mellen has uncovered and points to why “Hellman and Hammett” is unique: It’s the first book about either one written after free access to Hellman’s papers--in other words, the first book on either one that Hellman couldn’t control.

The result would appear to support Mary McCarthy’s famous remark about Hellman: “Every word she writes is a lie, including ‘a’ and ‘the.’ ” McCarthy was probably being generous in allowing her sole authorship of those words; Mellen’s book suggests that the playwright was a liar of almost insane proportions on nearly every aspect of her life.

She began with the myth of Sophronia, a black woman who worked briefly for the Hellman family in Louisiana when Lily was a baby. By the time she was an adult, Sophronia had become the key figure of her childhood--perhaps the most important female in her life. (She despised her weak, easily dominated mother.)

Yet Hellman’s most successful scam, according to Mellen, was passing off the “Julia” segment in her memoir, “Pentimento,” as fact. The story, which concerns Hellman’s mission to Austria before World War II to help a childhood friend and smuggle cash past Nazis, was made into a motion picture that won several Academy Awards in 1977. (Vanessa Redgrave played the title character, referred to by the New Yorker’s Pauline Kael as a “saintly Freudian Marxist queen”; Jane Fonda, who played Hellman, introduced her on stage at an Oscar tribute in 1976.)

Julia and her story, it turns out, were products of Hellman’s fertile imagination. The story might have been accepted as a first-rate work of fiction, if Hellman hadn’t insisted to her death that it was true.

There’s something in “Hellman and Hammett” to outrage nearly everyone. Conservatives will be angry at how principled Dash’s stand against McCarthyism was, and how unjust his imprisonment for refusing to name names. (Hammett comes off much better in this period than Hellman, who, though she also named no names, “was hardly worthy of the mantle of Joan of Arc she assumed after her ordeal was over,” according to Mellen.)

Literary leftists, meanwhile, will be angry at the dismissal of two heroes as “Roosevelt Bohemians.” And Hammett cultists will be upset for not having Lillian Hellman to kick around anymore. Hammett, it seems, was flaming out as a writer well before he met Lily. Mellen shrewdly observes that “Hammett’s aversion to introspection made him unsuited to transcend the detective genre,” though she is wrong, as many are, in labeling him a “mystery” writer. In some of his most famous books, “Red Harvest” and “The Maltese Falcon,” for example, there is virtually no mystery to be solved; “crime novelist,” perhaps.

Feminists, too, lose something: If you believe Mellen’s account of Hellman as myth-mongering, venal and hypocritical, she will never again be available for display as an early feminist icon. No woman who models her own personality so completely after a man’s--Mellen continually refers to the tough-talking, heavy-drinking Lily as a “She-Hammett”--could deserve such an honor.

About the only readers who won’t be enraged by “Hellman and Hammett” are those with no ideological ax to grind. They will put down the book more fascinated than ever by the story of two people who were so much in love for so long--mainly because they might have been the only two people on earth who could have possibly loved each other.