NEAR THE BEGINNING of this grimly fascinating book, in March 1975, some Americans in the Central Highlands discuss the cutting of the highway to the coast. They decide there's nothing to worry about. In Vietnam the roads are always being cut. This particular road has been cut many times before. Always government forces have recaptured it. At worst there will be no gas, no fresh fish for a while. This is on Thursday. A week later they look out the window and see "the guys with the red stars on their caps." It will be eight months before they are repatriated.

What David Butler's The Fall of Saigon conveys best of all is the sense of surprise, shock and panic among the Americans in South Vietnam as the Thieu regime collapsed around them. Many found it hard to believe "the party was over." The sudden and catastrophic transition of South Vietnam from an army with a country to a country without an army has been told before, and better, by Frank Snepp in A Decent Interval and Arnold R. Isaacs in Without Honor. Indeed, you could say Butler plays Walter Lord to Isaacs' Thucydides. All these writers of course have had to compete with the unforgettable television images of the debacle.

Butler draws on his personal experiences in Vietnam as a reporter for NBC News and an editor for Playboy magazine. The adventurous journalistic life he led colors his writing. Sometimes he is his own hero, a sort of Mel Gibson in The Year of Living Dangerously: bar girls, corruption, rock 'n' roll, drugs, perversion, decadence. Other times he cuts and pastes from memoirs and official reports. Many reporters will question the accuracy of conversations recorded here a decade after they occurred. These objections fade before the human pathos Butler vividly describes. His emotions, and ours, oscillate from shame at America's abandonment of her ally to outrage at the pusillanimity of the South's generals. We want desperately to find someone to admire as the "other side" rolls invincibly on toward the "Great Spring Victory."

In the end, there is hardly anyone to admire except the overworked helicopter pilots. It is astonishing there wasn't a massacre as the Americans fled. Surely Graham Greene was right, when he wrote in The Quiet American (1955), "'You and your like are trying to make a war with the help of people who just aren't interested . . . They don't want to be shot at . . . in five hundred years there may be no New York or London, but they'll be carrying their produce to market on long poles wearing their pointed hats (and) small boys will be sitting on the buffalos."

As Butler follows the paths of several Americans and Vietnamese through the alarms and disasters of the last weeks, he repeats a litany of calamities. In early March the Central Highlands fall to the enemy, a military disaster unprecedented in 20 years of war. In Saigon the Agence France Presse bureau chief is murdered by the police for his report of defections among Montagnard troops. To the north, along the DMZ, at Hue, at Danang, the morale of South Vietnam's army cracks, and entire formations disintegrate. At Danang, the evacuation is accompanied by horrifying scenes of anarchy and desperation. Along the coastal highway pathetic columns of refugees lurch southward. In Saigon President Nguyen Van Thieu exhorts his troops and, like French Premier Paul Reynaud in 1940, pleads for clouds of American airplanes.

AMERICAN Ambassador Graham Martin, a benign figure in this account though not in others, preaches calm. Fifty thousand persons, 5,000 of them American, are evacuated by air. A giant C5 transport crashes, killing 135 persons, including 76 orphans. A defecting South Vietnamese Air Force pilot bombs the presidential palace. On April 25 Thieu resigns and flees the country, and a government of national reconciliation led by "Big Minh" takes office. It is ignored by the communists who, however, allow the Americans to continue their evacuation from Tan Son Nhut Airport. When the airport at last comes under fire, choppers lift 1,300 Americans and 5,600 Vietnamese from the American embassy roof to Navy ships standing offshore in the South China Sea. In the confusion many thousands of South Vietnamese promised protection and refuge are shamefully left behind. So is the body of Marine Corporal Charles McMahon Jr., the last American killed in Vietnam.

In chronicling these events, Butler adds many seductive details, ranging from the evacuation of the ambassador's pet poodle to a number of very poignant goodbyes, of which this one example:

"For the past few years," he writes, "Mel Chatman (an Agency for International Development officer) had made it a point to get to Saigon at least one day a month for a tutorial in Vietnamese antiques from a man named Huon Van Sen. Mr. Sen was a former curator at the National Museum; in retirement, his collection of porcelain bowls and vases and other artifacts of Vietnamese culture rivaled that at the musuem.

"On Wednesday, the twenty-third (of April) Chatman had a driver take him out to Gia Dinh, to the house of polished teak that Mr. Sen had had transferred plank by plank from the delta. . . . 'Respected teacher,' Chatman said, 'I think you might now consider leaving Vietnam.'

"Mr. Sen looked the mandarin, with his elegant soft old clothes and wispy beard. 'I am seventy-two years old, nephew,' he said. He smiled. 'If I am going to die, I shall die in the land of my ancestors.'

"Then he gave Chatman autographed copies of each of his several books on Vietnam antiquities. Chatman knew the subtle formulas with which one constructed a modest speech that feinted and dodged around but never uttered the words 'thank you' and made to leave.

"The old mandarin, who well knew how sensitive Chatman was to Vietnamese sensibilities, nevertheless felt it necessary to give him a parting piece of advice. 'It is very dangerous time now for all Americans,' he said. 'Be careful in everything you do. Stay off the streets.'"

Butler works hard to set down every detail of Saigon's fall. He shows clearly that the city could never have been an Asian Stalingrad, defended to the last brick and the last man. He also communicates very well the charm this fabled city exercised on its foreign visitors. In Butler's best pages the tree-lined boulevards and the pastel-colored buildings newly washed by the early monsoon rains are only mildly intruded upon by a glimpse of flares before sunrise and the distant sound of artillery fire. It must have been a very hard place to say goodbye to.

Reid Beddow is an assistant editor of Book World.