THE JAPANESE TODAY Changes and Continuity By Edwin O. Reischauer Balknap/Harvard University Press 426 pp. $25

FOR MANY YEARS Americans headed for careers in Japan have been preparing themselves with the books of Edwin O. Reischauer. Few people know the subject better than he. He was born in Tokyo as the son of missionaries and passed his childhood there. He spent decades in the academic field as student, teacher and author. For five years he was the U.S. ambassador to Japan. Today, he is the reigning eminence of Japan studies at Harvard University.

Reischauer has occasionally stirred up controversy in Japan, notably in 1981 with an interview in which he acknowledged that U.S. naval vessels calling at Japanese ports had nuclear weapons aboard. The Japanese government has always done its best to keep this issue shrouded in fog. But in general that is not Reischauer's style.

Yet, for all his stature, the question arises: Have the times overtaken Reischauer and his view of Japan? He stands at the core of a graying generation of American specialists who love Japan deeply and, their detractors maintain, have too often been willing to excuse its faults. As the country recovered from the devastation of World War II, they were on hand both to counsel Japan and urge the outside world to show patience with its many foibles.

Today Japan has emerged undeniably as an economic superpower. It is the world's most envied industrial producer and its biggest creditor. The Japanese are buying companies and real estate helter-skelter in the United States. They are underwriting much of this country's national debt and giving it a run for its money in virtually every field of high technology. Isn't it time, many people are asking, for Japan to begin "living up" to its responsibilities in the new world order? And for an end to the excuses for shirking it?

Now Reischauer has reworked an introductory book he brought out in 1977, The Japanese, to make The Japanese Today: Change and Continuity. The revisions -- and they are fairly extensive -- focus on economics and foreign relations. They are his efforts to get his well-read primer up to date.

The reader, however, will not find a real shift in perspective. Reischauer at age 77 remains fundamentally a friend of Japan and its people. He offers praise in good faith, making a strong case that Japan's economic success is due for the most part, not to market closure, predatory trading practices or the "free ride" it gets from the United States in the form of military protection, but to the sweat of its national brow.

As a good many historians do, he looks to the feudal era for the origins of today's economic prowess. Even before Commodore Matthew C. Perry's "black ships" arrived in 1853 to force Japan out of two and a half centuries of isolation, the country had built an advanced pre-industrial economy, with huge cities, a large agricultural surplus, a banking and credit system, a respect for education and a potential entrepreneurial class, the samurai. The addition of Western technology and institutions provided the boost for a take-off.

He traces the remarkable story of how a feudal people created modern affluence in a few generations, then lost it all through the suicidal aggression of World War II. Never ones to sit by idly, the Japanese got back on their feet immediately after the 1945 defeat, rising this time to challenge the United States and Europe through economic output rather than armed force.

Reischauer devotes much space to how the Japanese stress the group, whether it be at school, work or recreational outings like the mass climbing of Mt. Fuji that takes place every summer. While foreigners smirk at it, Japanese see group activity as the key to personal fulfillment. Ideally, Reischauer writes, the Japanese likes to see himself as "the product of firm inner self-control that has made him master of his less rational and more antisocial instincts . . . social conformity to the Japanese is no sign of weakness but rather the proud, tempered product of inner strength."

Thinking of this sort can work well for a national economy. As Reischauer points out, managers in Japan look for long-term benefit for shareholders and employees alike, not high quarterly profits. Workers see themselves as part of the company and its prosperity as inseparable from their own. Government and business view each other more as partners in national development than adversaries.

Given both the depth of international anger with Japan and the author's knowledge and experience, the reader can't help wishing there was a bit more advice on the pages about solutions. Reischauer notes faults on Japan's side but never really offers strong criticism nor substantive suggestions on how things can be changed to bring about the progress in its international relations that he predicts is coming. (One oddly included exception, however, is a detailed proposal for how Japan could improve its teaching of English.)

THE BOOK's strength is in introducing the fascinating subject of Japan to newcomers. In classic lecture room form, it begins with discussions of geography, natural resources and agriculture. It goes on to a fast-forward viewing of 17 centuries of recorded history, then offers a look at contemporary society, politics, economy and foreign relations.

Given his academic background, Reischauer's writing is generally brisk and accessible to the lay reader. In places, he is able to sum up, in an aptly crafted sentence or two, some elusive reality of the Japanese system. Of the emperor and the "imperial will" that supposedly guided the country through its pre-war modernization, he writes:

"The Japanese leadership was able to combine an extreme reverence for the emperor with a complete willingness to force decisions on him regardless of his own wishes . . . . The politicians felt that the 'imperial will' was to be discovered through the voice of the people as shown in elections. Militarists and extreme nationalists believed that only they understood the 'imperial will.' No one thought of asking the emperor himself."

Or, concerning the homogenized mass media: "Tens of millions of Japanese, intellectually armed with the same television and newspaper news and opinions, sally forth to work each day with the same facts, interests and attitudes."

Unfortunately, individual Japanese rarely come to life on Reischauer's pages. He prefers generalizations, in contrast to other survey books on the subject that have appeared in such numbers in recent years. Americans embarking on a study of Japan are likely to find this book as they have Reischauer's past works -- informative, but a bit of a trial to get through.

John Burgess, a staff writer for the Business section of The Washington Post, reported from Tokyo, 1984 to 1987.