Rethinking warfare; National Defense, by James Fallows. New York: Random House. $12.95.

Spending more money on the military doesn't necessarily produce a stronger defense -- unless you spend it wisely. In this important book James Fallows offers though-provoking suggestions on how to spend it -- and how not to. And on what other steps should be taken to beef up American military strength.

In the process he torpedoes one of the most cherished icons of the military: that the more expensive and complicated a weapon is, the more it will help our nation's defenses.

The opposite is likely to be, he insists. He urges Americans to demand weapons that are lighter, more maneuverable, and cheaper. That way the nation can afford to buy more of them, Fallows says; they don't break as often, and there's more money left in the budget to give soldiers the all- important training in their use. In most cases, the weapons perform better -- and the military is stronger.

His book comes just as CBS-TV has presented a five-part series on the "Defense of the US," with jolting sequences on nuclear war. CBS said its hope was to spark a national debate on what should be done to improve American forces.

CBS and Fallows agree the real question is not more money vs. less, but how the money is used.

Fallows argues that radically different thinking about military issues is necessary before more weaponry is purchased, if the military truly is to be strengthened.

More important than exotic new weapons, he says, is improving troop morale, and the military's rapport with civilians. Plus giving commanders more authority, devoting more effort to training, and using "hard-headed skeptical reason" to cut through "the theology of nuclear weapons."

The one thing about war of which you really can be certain, he warns, is that it's unpredictable. Thus he stresses the necessity of developing a military equipped in weapons and training for maximum flexibility.

Even as "National Defense" is published, Congress and the Reagan administration are in the process of increasing the US defense budget by billions of dollars, without public evidence of the radical rethinking Fallows demands.

In the nation's capital the major defense debates instead appear likely to revolve around how much money should be spent, and on which of three sophisticated nuclear-bomb "carriers" the US should concentrate.

Fallows has researched his subjects well. Some issues he raises are not new -- particularly his recital of the shocking deficiencies of the M-16 rifle. But even here he performs well a major journalistic function: to provide perspective by bringing together all information which bears on an issue.

The book is clearly organized and written. It presumes prior knowledge neither of military issues nor of terminology, only a reasonable degree of interest in the nation's future. It will m ake most any reader think -- which is its aim.