An NYC native and former Post and Daily News editor, Pete Hamill knows a lot about the city's history. And in Downtown, he's gonna make sure you know it. Assuming the guise of a PBS documentary, he starts at the southern tip of Manhattan and walks uptown, pausing at various points to conjure up the distant past. This results in such unbelievable passages as ''Standing at the iron fence of today's Bowling Green, I sometimes imagine Willem Verhulst....'' When Hamill breaks free of these reveries with gorgeous riffs that detail his excursions in Greenwich Village four decades ago, we get a small glimpse of what Downtown could have been. Instead, the book merely resembles Ken Burns lite.