JUDGING BY THE remarkable public response, Nicholas Lemann’s new book, The Promised Land:   The Great Black Migration and How it Changed America, [1] could prove to be the most important piece of nonfiction on the urban ghetto since Daniel Patrick Moynihan penned his controversial report on the black family in 1965. Lemann’s 362-page opus, which traces the consequences of the 1940-1970 migration of five million southern blacks to the urban North, achieved the unlikely status of a bestseller within weeks of publication. The New York Times Magazine, Atlantic Motlthly, Los Angeles Times, and Washington Monthly all ran lengthy excerpts. Reviewers gushed over the book, despite the fact that accounts of America’s race problem usually provoke diverse, if not divisive, reactions. Hip Esquire magazine concluded that “thanks to Lemann, white America will never be able to think about the ghetto poor in quite the same way again.” Both left-leaning essayist Gary Wills and conservative columnist George Will could barely contain their enthusiasm, too; the former labeled Lemann’s account “indispensable” and “brilliant,” while Will confessed that he had “never learned so much about contemporary America from a single book.”