N-bomb inventor: idea won't work; The Truth About the Neutron Bomb: The Inventor of the Bomb Speaks Out, by Sam Cohen. New York: William Morrow & Co. 226 pp. $12.50.

This summer will mark the 25th anniversary of the idea for a neutron bomb - a concept presidents, legislators, and Pentagon planners tossed around like a radioactive potato until it landed in the lap of Ronald Reagan.

The current President ended the decades of debate two years ago by deciding to build the N-bomb, because the administration believes it can be used to protect NATO from a conventional attack by the Warsaw Pact.

Unfortunately, says N-bomb inventor Sam Cohen in ''The Truth About the Neutron Bomb,'' that idea won't work.

Building the neutron bomb is an inadequate solution to a more serious problem , Cohen argues. America's NATO partners lack the political will to match Warsaw Pact conventional forces; they instead depend on the US nuclear arsenal to deter aggression. This has led to a NATO strategy that calls for the possible use of tactical nuclear weapons in response to a conventional Warsaw Pact attack.

But, Cohen insists, the West has no doctrine on using these tactical nuclear forces, which will now include N-bombs on Lance missles and in 8-inch artillery shells. Because our allies don't want N-bombs on their territories, they will be stockpiled in the United States. However, by the time they could be deployed in response to an invasion, Western Europe would be in Soviet hands.

Even worse, says the the former Rand scientist and Pentagon weapons planner, the NATO alliance is blind to the realities of Soviet military thinking. It assumes the USSR would permit the West to implement its first-use policy, when actually Soviet strategy is to neutralize NATO's tactical weapons in a preemptive strike that would precede a ground attack.

Cohen also infers from Soviet tactical writings that the USSR possesses neutron bombs itself and, unlike NATO, has a doctrine on how to use them in a European attack.

In his concluding chapter Cohen calls on the US to stop risking its own security in order to protect lazy allies. The US, he says, should withdraw from Europe and plan instead for its own defenses against the Soviet strategic threat.

In addition to developing these arguments, Cohen also exposes the way in which what he regards as a mistaken policy came to be. He describes the political chicanery that spanned nearly a quarter-century and involved all three branches of the government and a host of nuclear scientists. What emerges from his highly readable account is a story of clashing egos formerly hidden behind the veil of national security.

Cohen answers moral opposition to the N-bomb with the amoralism of Realpolitik:

Existing fission and fusion bombs, he explains, obliterate entire cities with their powerful blast and incredible heat, and they also leave behind fallout that would be lethal to people not killed by the actual explosion. In fact, an A-bomb blast would leave vast areas of land uninhabitable for years.

If an N-bomb were exploded at a sufficient height, on the other hand, its blast and heat would be dissipated in the atmosphere, while its lethal neutron radiation would kill all life below. Yet there would be no fallout, and radiation would drop to safe levels in hours. Civilians could be shielded from the bomb's effects. Its yield could be limited, in theory, to a few city blocks or a military base.

In theory, several kinds of neutron bombs are possible. Cohen favors development of a pure fusion weapon with an output of only one-hundredth of a kiloton (a minuscule fraction of a Hiroshima bomb), nearly all of whose energy would go into neutron radiation.

Called by the Soviets a ''capitalist weapon,'' because it leaves structures intact while killing people, the neutron bomb could have important military applications; it could knock out a tank attack in heavily populated Europe, or flush an enemy from an occupied city.

Cohen doesn't deal with the moral objections of those who argue that the N-bomb's political unacceptability must ultimately be the decisive factor, rather than its military advantages. These opponents point out that the N-bomb could make use of nuclear weapons seem much more attractive than at present. They also point out that strategists feel the Soviet response to an N-bomb attack is unpredictable; it could result in a large-scale exchange of fission and fusion weapons, producing a nuclear holocaust.