Critics of affirmative action say that it is unfair to black students to be forced to compete against whites who are better prepared for demanding academic work. Some of the evidence collected by Bowen and Bok confirms this; in less selective institutions, black graduation rates six years after entering college are significantly lower than white graduation rates. Black students nearly always perform less well than white students, and also perform below the levels predicted by their SAT scores. A chapter in the Jencks and Phillips collection calls this ''disturbing'' and adds that ''most sobering of all, the performance gap is greatest for the black students with the highest SAT's.'' A co-author of that chapter is William G. Bowen. Still, Bowen and Bok conclude that the overall picture proves that minority students are not ''overmatched'' in comparison with whites admitted with much higher SAT scores to the nation's top schools. The picture improves even more if one examines the years after college. Despite their lower SAT scores, black graduates of the nation's selective colleges are active participants in civic life. They report high degrees of satisfaction with their experiences in college.

In their most impressive finding, Bowen and Bok show that of the 700 or so black entering students from the class of 1976 who would not have been admitted to one of the nation's more selective institutions had strictly race-neutral criteria been applied, 225 obtained professional or graduate degrees, 70 became doctors, 60 became lawyers, 125 became business executives; and as a body, they earned an average of $71,000 annually. Bowen and Bok interpret these facts to mean that an increase in the size of the black middle class justifies racial preferences. They may well be correct. There is no more important step to be taken along the road to racial justice than building and strengthening a black middle class. Every African-American who enters a profession or buys a house in the suburbs gives the lie to two pervasive cynicisms -- one that blames black Americans for their own inequality and the other that in blaming white racism for all the ills of America ends up excusing self-defeating black isolationism.

But it would be wrong to conclude from ''The Shape of the River'' that affirmative action works. What Bowen and Bok have proved is that going to a top college works. Their book unintentionally fuels rather than quenches the passions over affirmative action. For if a degree from a top college benefits those who receive it as much as Bowen and Bok clearly demonstrate, then those passed over for admission to those colleges really do have cause for complaint.

And because Bowen and Bok's data are limited to the more selective institutions, they have little to tell us about the fates of minority students who never make it to the level of applying to those colleges. The material assembled by Jencks and Phillips helps explain why that group is so large. A gap between blacks and whites on intelligence tests appears when children are 4 years old. By the age of 6, black vocabulary scores match those of whites who are 5. By the age of 17, black scores are equal to those of white 13-year-olds. This means that African-Americans who show up in the Bowen and Bok study have already won some of life's biggest battles. By scoring in the 1200 range on SAT tests, they are most likely either middle-class already or will push themselves into the middle class through their determination and effort.

The real problem arises among those black high school graduates who never fully recover from their initial disadvantage in testing and who therefore wind up scoring in the 800-1000 range on SAT's. The best of these students will attend colleges that are somewhat selective, and which therefore still exercise some degree of racial preference in admissions. But while the preference is smaller than at the most selective colleges, the impact on many students is larger (Thomas Kane's data indicate that black and Hispanic students receive an 8 percent to 10 percent preference at the most academically selective fifth of four-year institutions, but only a 3 percent preference at schools ranked in the fourth of the five tiers). Getting into and graduating from one of these colleges may well play a more significant role in the life prospects of a medium-range SAT scorer of either race than graduation from a top college plays for a high scorer of either race, for these are the colleges that historically made it possible to move from the working class into the middle class. The benefits gained by minority students at the top colleges, in other words, could come at the price of greater conflict between black and white applicants to those less selective colleges where middle-class aspirations meet head on.