The first glimpse inside the SALT talks; Doubletalk, by Gerard C. Smith. New York: Doubleday & Co. $17.95.

During this year's presidential campaign, the three leading candidates have come to agree on the necessity of SALT III. Whether acting from conviction or expediency, each reached the same conclusion: a continuuing dialogue with the Soviets on arms control is safer and wiser than a total breakoff of communication.

John Anderson has been saying this all along. President Carter initially withdrew his support of Salt III after the Soviet invasion of Afghanistan, but later, sensing shifting winds of opinion, he said he would press for congressional approval of that agreement early in 1981 or even sooner. And just 16 days before the election, Ronald Reagan, while still urging that a "flawed" Salt II be scrapped, called Salt III a high and urgent priority.

Now Gerard Smith, who led the Salt I delegation during the years 1969-72, has written a case book that makes clear the reasons for this consensus and offers a most important account of how the arms limitation agreement was reached.

His book, "Doubletalk," shows that credit should go to Richard Nixon for seizing the opportunity. Nixon perceived tht both Washington and Moscow shared a need for some measure of control over an arms race that was overtaxing resources and threatening overkill. And Nixon believed that he, a known hardliner toward the Soviets, could sell any resulting treaty to the nation.He was right.

For 2 1/2 years, in alternate rounds at Helsinki and Vienna (with gaps between for consultation), the Soviet and American delegations sparred, circled, feinted, and took each other's measure. They finally achieved about 90 percent of the ultimate agreement -- a treaty limiting anti-ballistic missiles in both sites and numbers, and also putting a freeze on long-range missiles. Along the way there were ancillary breakthroughs: an improved hotline using satellites, and new precautions to prevent accidental or unauthorized launches.

The last phase, as Smith tells it, shows us President Nixon, along with Secretary of State Kissinger, in a less enlightened role than at the start. As early as March 1971 they had established a "back channel" of negotiations via Soviet Ambassador Dobyrnin in Washington. While not questioning the President's right to do th is, Smith makes it clear that such activity -- of which he was often informed long after the fact -- made his task infinitely more difficult.

The negotiations came to the boiling point in the spring of '72. Because it was an election year, Nixon wanted a triumphant finish. A summit meeting scheduled for May in Moscow provided the perfect setting. There would be no doubt, ever, to whom the credit should go.

As the deadline neared, both front and back channels were busy. Nixon and Kissinger arrived in Moscow; Smith and his team were left stranded in Helsinki. Kissinger and two non-technical aides made broad sweeps over highly technical matters of which they were largely ignorant. Smith and company tried desperately to keep abreast of developments and of top-level meetings at which memoranda of conversations are rarely kept.

The micracl is that something constructive did emerge: "While the product of the first SALT dialogue was perhaps more modest than reflected in the triumphalism which the Nixon administration resorted to . . . . it was a good first fruit and a product of which the two delegations can be proud," writes Smith. Taking a longer view, he notes that a process had been started "of talking seriously about issues on which the survival of our two societies and the rest of civilization depends."

"Doubletalk" offers a fascinating glimpse of the high-level give-and-take, but it is not easy reading. Technological terms and endless abbreviations for agencies and titles slow down the narrative. Smith is also quite capable of using such terms from the bureaucratic quagmire as "analogized" and "maximalist positions."

But some readers won't mind. The heart of the book -- the message that each of the three presidential candidates has come to accept, is that communication with the Soviets is sfer than silence. However diverse the global aims of the USSR and the US, it is still better to have delegates from both countries talking across a conference table. SALT I, the hangfire SALT II, and the proposed SALT II are all part of a vital continuum.

We are fortunate that one of the people of the forward lines of these negotiations has told his important story.