BOOK REVIEW / FICTION : Money, Greed, Secrets From Vietnam : THE CODICIL <i> by Tom Topor</i> , Hyperion, $21.95, 352 pages

If history is a nightmare from which we struggle to awaken, as James Joyce once observed, then the recurrent nightmare of the American dreamer is Vietnam.

“I used to see faces . . . in country--special forces guys, or ARVN [Army of the Republic of Vietnam] mercenaries, or doorway gunners--people who really love to kill,” recalls one history-haunted veteran in Tom Topor’s “The Codicil.” “People who love to make mama-sans beg for mercy before they gut-shoot them.”

“The Codicil” is the story of Matt Marshall, a Vietnam vet who returned from the war to his wife and children with a dark secret and a driving ambition to make money.

Years later, when he dies suddenly of a stroke, Marshall leaves behind $100 million--and a mysterious codicil that bestows half of his fortune on a nameless child he supposedly fathered while in Vietnam.

Marshall’s family--and, notably, their greedy lawyers--hire an investigator named Adam Bruno to find out whether the child actually exists.

Fearful of losing half of the legacy, they hope he doesn’t try too hard, but they’ve got the wrong man: Bruno is one of those heart-of-gold private eyes who populate hard-boiled mysteries, a man driven by righteous rage and a yearning to see justice done.

So Bruno starts reeling back the years, searching out Marshall’s war buddies and ripping open old wounds, sometimes with bloody consequences.

He finds his way to a series of colorful and exotic characters who fought alongside Marshall in Vietnam, including a bloodthirsty-grunt-turned-whacked-out survivalist, a field medic who performs abortions, a mysterious Vietnamese officer named “Ghost,” a medevac helicopter pilot nicknamed “Angel” who is reduced to giving traffic reports on the radio, and various other victims and survivors of Vietnam.

Bruno is a down-on-his-luck lawyer who has reinvented himself as a private investigator after an awkward encounter with the New York bar, and it is much to Topor’s credit that Bruno’s new career is depicted in all of its sometimes dreary routine.

Bruno may have mastered the art of detection in the age of the Internet--he burns up more on-line minutes than shoe leather--but the routine work of the investigator has not changed.

“You check . . . records,” Bruno observes. “Court records . . . voter registration lists . . . utility companies . . . commercial licenses. . . . Whatever you want to do in this society--hunt, or fish, or massage, or marry, or sell dogs or dirty videos, you need a license to do it.”

But Bruno’s legwork and Net surfing don’t yield any secrets that really matter, and the author relies on a deus ex machina for virtually all of the crucial revelations in the story--an anonymous source who uses the code name “A. Nonni Muss” and sends tantalizing bits of evidence to Bruno from time to time by Federal Express.

Topor has adopted a certain multiculturally correct stance in “The Codicil.”

For example, the honest, endearing and competent figures in his story include a black prosecutor and a Latino-Asian translator, both women, while the rich white people who attached themselves to Marshall are generally boorish, inept, greedy and corrupt. And yet Topor’s detective hero is capable of describing Nicole Maldonado, daughter of a Puerto Rican father and a Vietnamese mother, as “the great argument for miscegenation I’ve ever seen.”

When, like Topor, a novelist is also a playwright (“Nuts”) and screenwriter (“The Accused”), there’s always a risk that the novel will be not much more than a treatment for the author’s hoped-for movie deal.

And it’s true that “The Codicil” is a plot-driven tale that may twist and turn like a roller coaster but never jumps its track.

Still, Topor’s thriller is an earnest attempt to make sense of Vietnam, to measure what it did to the men and women who served there, to glimpse the heart of darkness and explain what makes it beat.

“You must have really enjoyed Vietnam,” Bruno says to one of the nastier villains of the piece, a former CIA operative with blood on his hands and a dark secret of his own.

“I enjoyed mattering,” says the otherwise detestable spook in a moment of self-justification that rings true. “Things were at stake. Big things, not just whether blackened snapper is on the menu or the location of your parking space.”

If every detective story is essentially the story of a quest, Topor’s book is a particularly lucid and compelling example of the genre.

Bruno is a knight errant--”lawyer, quester, and all-round early-morning man,” as he is described--and what he seeks is something as corny and old-fashioned as truth and justice.

And that’s what really matters in “The Codicil.”