Hanoi Spy Who Came in From Cold. UNEARTHING A MOLE

A SERIES of new books about a wartime secret agent who helped defeat the United States has caught the imagination of readers in Vietnam. The story of Vu Ngoc Nha, who influenced many key events as an adviser to South Vietnam's presidents, is being revealed in a trilogy of spy thrillers, 14 years after the end of the war.

By unveiling Mr. Nha's role as a deep-penetration ``mole,'' Hanoi has for the first time divulged information about its former spy rings, reflecting a new openness about its side of the war.

The first book, called ``The Adviser,'' was published last year, reaching an unusually large run of 120,000 copies. The next volumes, ``The Dragon Palace'' and ``The Chameleon,'' are due out later this year. In addition, Nha's life will be featured in a film, Vietnam's first in color and its longest.

Posing as a pious Catholic, Nha was recommended to high positions in Saigon by gullible church leaders. But he was twice revealed: first, by an informer in 1958 and then again in 1969 by US intelligence officials.

Yet, he was so convincing that his disloyalty was never quite believed by his anticommunist bosses. As a result, he was able to return to power after spending, in all, seven years in jail.

``To us, he was a strong hero because he worked with the other side for two decades,'' says Huu Mai, who wrote the trilogy and the screenplay on Nha.

Nha joined the Communist Party as early as 1950 and became chief of the communist's spy network in Saigon. ``He was half-communist and half God's worker,'' says Mr. Mai.

Born to a northern Catholic family, he fought against French colonizers and then was sent south as an agent. Nha eventually became close to President Ngo Dinh Diem, who was a devout Catholic. After Diem was killed in a 1963 coup, Nha became an aid to President Nguyen Van Thieu.

As a spy, he both influenced decisions and tipped off the communists to enemy plans. After his role was revealed, the church defended him. Many Catholic friends said his subsequent confession was just a test of his faith, says Mai. Nha was seen as a kind man, leading a civic-minded life. To his friends, the guilty party was the US for accusing him.

Nha secured such official secrets as plans to corral peasants into ``strategic hamlets'' and to assassinate communist leaders in the south, says Mai. He also influenced noncommunist anti-US activists.

Perhaps his most effective gambit was convincing Thieu to allow South Vietnamese soldiers to take a few days off for the Tet (lunar) new year in late January, 1968. With defenses down, the cities were easy targets for a decisive communist offensive.

Today, Nha holds the rank of colonel in Army intelligence, says Mai. His whereabouts are held secret.

``He needs to be protected,'' Mai says. ``People who gave him information still cannot imagine he was a spy.''