WHAT explains the urban revival of the 1990s? Cen- trists and conservatives have credited the innovative crime and welfare policies implemented by a new generation of take-charge mayors, most famously New York’s Rudy Giuliani, who ceased looking to Washington for help.  Many liberals, by contrast, have minimized the revival, while holding fast to the idea that only a more powerful federal government or the creation of strong regional governments can truly save the cities. A different, bottom-up liberal perspective is offered by Alexander yon Hoffman in Block By Block, House By House, † a richly detailed reportorial account of local activists in New York, Boston, Chicago, Atlanta, and Los Angeles. Von Hoffman, a senior research fellow at Harvard’s Joint Center for Housing Studies who has spent years following and studying community-development groups around the country, celebrates “the rebirth of America’s urban neighborhoods,” but claims that the neighborhoods themselves, not the new mayors, deserve the lion’s share of the credit.