During a brief visit to San Francisco today and I found myself with 10 minutes to spare before Bill Stout's book shop opened its doors. So, I walked to the Embarcadero Center to take in one of my favorite urban spaces. Bisecting the Center are 3 levels of public promenades, or walkways, 2 of them suspended between the Center's retail and office tower buildings. The public ways cut through the center of four blocks and work their way down to Embarcadero Plaza. Canted facades o