Starting Hot, Ending Heroic : SHINING THROUGH<i> by Susan Isaacs (Harper & Row: $18.95; 402 pp.) </i>

A novel is like a love affair. It can start hot and fizzle or it can begin slowly and build into something majestic if you’re willing to stick with it. Few affairs or novels start hot and stay hot. “Shining Through” by Susan Isaacs falls into that rare sizzling category.

The necessity of moral and physical courage forms the core of the novel. The surface, seducing the reader ever deeper into the story, is a good old-fashioned love triangle. Linda Voss, star secretary and spinster (at 31 in 1940), falls in love with her gorgeous blond boss, John Berringer. He is a Wall Street lawyer, a WASP whose features are so sharply perfect that if you bumped into him you’d bleed.

John’s wife, Nan Leland Berringer, the daughter of the very senior partner of the law firm, unceremoniously dumps him. John, on a rebound so high he could play in the NBA, marries Linda. Beautiful John pines for Nan while half-Jewish Linda (her secret) loves John.

To add to the complication, Linda, thanks to her skill with working-class German, begins secretly working for Edward Leland, Nan’s father. Edward, a hero in World War I, has clandestine and important tasks as the United States stumbles into World War II.

The novel’s voice is Linda’s: no-nonsense and with a keen sense of how the world really works. For example, her father was Jewish. Her husband doesn’t know. She says, “Granted, John didn’t seem like the prejudiced type, but how many people are there in the world who jump for joy over a Jew? Not that he, or anyone else in the office knew. I wasn’t an idiot. Wall Street law firms didn’t hire Jewish secretaries (to say nothing of lawyers), so since I didn’t look it, why volunteer?”

After Pearl Harbor, Linda, John and Edward Leland move to Washington to work with the organization that will become the OSS. Linda acts as Edward’s secretary, and as the Vichy government denies public service work to Jews, as the Nazis’ Rassenschande (race defilement) policies begin to reach the secret corridors of D.C., Linda realizes her people are threatened.

Not that she was reared as a Jew, not that she follows Jewish religious practices, but according to the Nazis, she’s a Jew.

Her moral awakening coincides with an awakening of another sort, more personal and immediately painful. Nan, she of the flawless features and advanced decorating taste, has grown bored with husband No. 2. She doesn’t just land on ex-husband John’s doorstep, she lands in his lap. Linda walks in on them.

Betrayed by her husband, feeling betrayed by Edward Leland because he knows about his daughter Nan’s deception, Linda volunteers for a dangerous mission inside the Third Reich.

John, busy being torn between two women--one (Nan) with class and the other (Linda) with sex-appeal--wimps out of the decision. It is Edward, not John, who begs Linda to give up infiltrating the Reich. Her odds on returning from Germany are about the same as being hit by a meteorite.

Linda’s old life is like an old dress. It just doesn’t fit anymore. She doesn’t belong anywhere. Emotionally, espionage work is an escape from the unbearable tension of the love triangle.

But it’s more than that. Women aren’t allowed to fight wars; they’re just allowed to die in them. Becoming a spy is one way to fight. Every nation, no matter how sexist, needs female agents.

When Linda defies Edward Leland and shouts, “This is my war,” she means it. “When you’re so oppressed and depressed by your own life, you can cave in and show the world you’re the sucker they’ve always thought you were, or you can finally come out fighting and say: ‘Listen, you bastards, this is unfair; I won’t put up with it. I’m entitled to’ . . . And whammo! Out pops what you really want. Out pops the truth. And the truth was I wanted to fight just as much as every 18-year-old boy who’d up and enlisted the day after Pearl Harbor.

“Why not? I am as American as apple pie.”

Disguised as a commercial novel, accessible to a huge proportion of the reading public as opposed to a literary novel , “Shining Through” explodes more than one myth about being as American as apple pie.

Myth One: A citizen, corn-fed from Nebraska, is more of a real American than a Jew from New York.

Myth Two: Women are not as patriotic as men, or perhaps more accurately, their services are not as important as men’s services in time of war.

Myth Three: Women have no understanding nor do they care about military strategy or the military future.

Myth Four: Women are not willing to kill for their political, moral beliefs in time of extraordinary crisis.

Myth Five: What happens to a group of people in a foreign country is of no importance to an American.

Of all the myths, five is currently the most compelling. In retrospect even someone whose IQ hovers at body temperature can perceive the plight of Jews worldwide during the 1940s. However, what of the plight of blacks in South Africa? What about women in fundamentalist Islamic countries or homosexuals in Communist countries? If an American identifies with any of these groups and works for their relief, does that make that person less of an American? More of an American?

Linda Voss chose to do something for other people, people she felt were her people . “Shining Through,” with consummate commercial cleverness, lures the reader into asking some unsettling moral questions about her/himself and about our current political leadership.

Linda Voss found her answer. Read the novel and then find your answer, not for the 1940s but for today. What are you willing to risk for other, suffering people--your people?