CITIES are humanity’s hothouses, where human potential  develops to its fullest pitch of excellence and variety.” So says Myron Magnet, editor of the Manhattan Institute’s City Journal, a magazine that gets more policy bang per philanthropic buck than any of its think-tank publication cousins. And now Magnet has collected the magazine’s best in a single volume, The Millennial City: A New Urban Paradigm for 21st-Century America.