Brown's sensible ideas on defense; Thinking About National Security: Defense and Foreign Policy in a Dangerous World, by Harold Brown. Boulder, Colo.: Westview Press. 288 pp. $16.95.

The Russians have 40,000 tanks. The United States has 10,000 tanks. What should the US do to even the balance? This is the kind of nuts-and-bolts problem facing US defense planners - and US citizens - as the debate over security issues heats up and the 1984 presidential election draws near.

Harold Brown, who was Jimmy Carter's secretary of defense, jumps into that debate with this thin volume that is packed with insights on the most vital defense issues of our time.

Dr. Brown's book has several important strengths:

* It is, despite some tedious, professorial prose, easy to grasp. There's very little Pentagonese, like ASAT, MBFR, or VTOL.

* It is dispassionate. This is especially important, as the election draws closer, and hyperbole replaces horse sense.

* It is written with authority. Dr. Brown has the respect of liberals and conservatives, Republicans and Democrats, and he has studied defense issues from key vantage points both in and out of government.

* It is wide-ranging. If you're interested in stealth bombers, Reagan defense policy, Soviet naval strength in the Pacific, or Soviet tank forces in Europe, it's all here.

On those tank forces, by the way, Brown sees little hope that the US could - or should - match the Soviets tank for tank. He explains that just buying another 30,000 tanks would cost $50 billion. Manning those tanks would require another 300,000 US troops. But doing something is essential. There is now a nuclear standoff in Europe. If war ever broke out, it could very well be limited to conventional arms like tanks and artillery.

So what to do?The answer in this case, Brown says, lies in technology - specifically, antitank weapons. Laser-guided artillery shells and bombs, infrared imaging systems, air-launched missiles with submissiles guided by millimeter waves. The US must press such technology ahead, Brown says, if the Soviets are to be matched in tanks and other weapons at reasonable cost.

Are Americans and their allies still willing to pay the costs of their own defense? In Brown's eyes, the answer is unclear. So the outlook for protecting Western freedoms for the rest of this century is uncertain.

Obviously, the US and its allies have valuable assets on their side. They have rich resources, powerful economies, talented citizens, technological prowess, military skills, and the obvious material and spiritual benefits of democracy and free economic systems.

And the Soviets have all kinds of problems. They can barely feed themselves. Their economic growth rate is slowing. They are flanked by a billion hostile Chinese on the East and restive satellites on the West. Brown points out that if Soviet military forces were withdrawn from countries like Hungary and Czechoslovakia, their communist governments probably wouldn't last a week.

Even so, Brown is worried. In the United States, the postwar consensus that helped America firmly oppose communist expansion is weakening. In the 1950s Americans - without grumbling - supported the military draft as well as a defense budget that equaled 10 percent of the nation's economic output. Today there is no draft, and President Reagan's efforts to lift defense spending barely above 6 percent of gross national product have raised a political storm.

In Europe, the lessons of the 1930s are fading from memory. Neutralism, and a degree of pacifism, are emerging.

Furthermore, the West has some worrisome soft spots. One of those, Mideast oil, has become more and more vulnerable to a Soviet military thrust. Europe and Japan rely heavily on that oil, but expect the US to defend it almost single-handedly.

Brown emerges in this book as neither a hawk nor a dove. He warns hawks that boosting defense outlays too quickly could simply lead to higher prices, not higher numbers, for aircraft, tanks, and missiles. He cautions doves that US-Soviet strategic forces are about equal now, but ''the trend has been adverse.''

Brown doesn't claim to have all the answers.But his careful approach to the issues will make readers think. If you are interested in defense matters, ''Thinking About National Security'' would be a good way to invest seven or eight hours of your time.