Replete view of ethnic culture in America's melting pot; Harvard Encyclopedia of American Ethnic Groups, edited by Stephen Thernstrom, Ann Orlov, and Oscar Handlin. Cambridge, Mass.: Harvard University Press. $60.

Do you know who the Carpatho-Rusyns are? Or the Kalmyks? Or even the Wends? If not, there is no reason to feel ashamed, even if they are among your fellow-Americans (should you be American). This reviewer had no idea who they were either -- until he picked and chose his way through this remarkable encyclopedia. It makes good its publishers' claim to be "a guide to the history , culture and distinctive characteristics of the more than 100 ethnic groups who live in the United States." It is even more.

But first -- for those who are curious but cannot afford the book's $60 price tag -- or whose interest is not specialized enough to entice them into its more than 1,000 pages -- a word more about the Carpatho-Rusyns, the Kalmyks, and the Wends.

The Carpatho-Rusyns (of whom there are over half a million in the US) came originally from the Carpathian Mountains, once mainly within Hungary but astride the present-day frontiers of Poland, Czechoslovakia, and the southwesternmost corner of the USSR.

The Kalmyks are Mongolian Buddhists who found themselves cut off in the Russian Empire through the vicissitudes of history and for whom the Soviet Union in 1921 carved out an autonomous republic on the Caspian, just to the west of Astrakhan. There are about 900 Kalmyks in the US, most of them in New Jersey and Pennsylvania.

The Wends are a Slavic people whose homeland is in present-day East Germany, just west of the Polish border astride the River Spree between Cottbus and Bautzen. The Wendish migration to the US took place in the mid-19th century, mainly to Texas.

If these little-known ethnic groups sound esoteric, the encyclopedia also offers pages and pages of fascinating detail on the more obvious ethnic components of the US: English, Afro-Americans, Germans, Irish, Italians, French, Jews, Chinese, Poles, Hispanics, and the many others. But if the editors chose to include the Cornish under a separate entry, one wonders why not the Channel Islanders too?

Of the entries which this writer read in their entirety, he found the one on the French the most lively. And he was surprised to learn from it that the usually ethnocentric and culturally exclusive French are among the quickest of groups to merge into the American mainstream. For example, French immigrants have been among the first to recognize that the place to head for right away is the Sunbelt. It is French-speaking Canadians who have tended to cling more tenaciously to a separate identity, once they get here.

Of course, there is a timeliness about the appearance of the encyclopedia now. The blue-collar ethnic backlash against the national effort in behalf of the unusually disadvantaged blacks and Hispanics has both produced potentially disruptive tensions within American society and helped force the turn in the mood of the electorate which has sent to the White House a new president committed to radical change. This book will tell you more about ethnicity, and in greater detail, than probably anything else on the market.

Americans already understand it better than most other peoples -- certainly than their often superior European cousins. But there is still the need to learn more. The splendid essays on general themes in the encyclopedia -- Thomas Pettigrew on prejudice, for example, Mona Harrington on dual loyalties, or Harold J. Abramson on assimilation and pluralism, to choose a few -- should be "must" reading for anybody in a position of authority or influence who has the overall well-being of Ameri can society at heart.