The Vietnamese have celebrated Tet since 200 B.C. This lunar New Year is like Christmas, New Year's Eve, Easter and the Fourth of July combined into three days of feasting and general merriment. In 1968, the Viet Cong added something new: During the early moring hours of Jan. 31, the first day to Tet, some 84,000 Viet Cong attacked nearly every American military base and every city and town in South Vietnam.

The Tet offensive was a tactical failure for the Viet Cong, but the longterm consequences were quite different. The sight of Viet Cong soldiers running amok throughout South Vietnam -- for however short a time -- brought the war home to millions of Americans as never befroe. Public opinion turned irrevocably against the war. Tet marked the beginning of the end of American participation in Vietnam.

Tet 1968 is the focal point of John Cassidy's first novel, "A Station in the Delta." The protagonist is Toby Busch, a newly arrived CIA field officer in the Mekong Delta town of My Tho, who learns months in advance that the massive asault is in the works. But a feud with his immediate supervisor, among other things, threatens to prevent Busch from getting the word out. The book gives a realistic portrait of the pre-Tet climate in Vietnam. The setting and characters are believable and accurate. A retired CIA operations officer himself, Cassidy served a tour in Vietnam.

The best books about Vietnam have been written by former war correspondents and young veterans of the conflict wuch as Frances Fitz Gerald, Michael Herr, Tim O'brien and James Webb. These authors have written convicingly of the frustrations arising from that guerrilla war, including criticism of the U.S. military and the South Vietnamese army and government.

Cassidy -- who is a generation older than the average Vietnam veteran -- expresses a significantly different view of the war. Toby Busch has his troubles with the CIA establishment, but the overall picture Casidy paints of our clandestine service and its role in Vietnam is a positive one. The author finds fault with some South Vietnamese, but leaves the reader with the impression that our former allies were a herioc, long-suffering people who were dedicated to fighting communism and did a good job of it.

The Viet Cong are Cassidy's real viallains, but the author also has some unkind words for two groups of Americans: antiwar protestors and the media. "All these kids beating their breasts in the States give me a pain in the a --," a young Army officer announces in a bar, "because they don't know a goddamned thing about the world they're so shook up about." And a young woman reporter vowing to wirte a fair story, talks her way into being allowed to witness a clandestine mission, during which a brave South Vietnames saves her life. But instead of being grateful, the reporter writes a viciously slanted story calling the CIA "Murder Incorporated." Later on, another reporter gets punched in the mouth when he tells Busch's boss, Bill Voight, that draft resisters have the right idea.

"A Station in Delta's" message is that Tet was a clear military and psychilogical victory for the United States and South Vietnam. "The countryside,' Bill Voigt says, was "relieved of the burden of a ruthless insurgent army that had for years been hiden in its reluctant breast." The victory was won, but the message did not get through to the American public. "Out soldiers have won this battle and the war," Voigt tells the reporter he has punched out, "but you and your smart a -- friends, and your wiser-than-anybody editors, have lost it. And so very single death and every drop of blood through all these years, that you 're had so much fun showing on the screens with your snide remarks, it's been all for nothing."

Yet in spite of Cassidy's cheap shots at the media and the antiwar movement, he has written a readable, intelligent novel. The writing is precise, the plot hums along smoothly nd most of the dialogue and characterization is accurate and well drawn.

"A Station in the Delta" is a good companion piece to Graham Greene's masterful "The quite American," which tells of a young, idealistic CIA agent who fails miserably in Vietnam during the latter stages of the French involvement. This book shows a not-so-young, far-from-idealistic agent who fares better during the beginning of the end of the American involvement there.