John Allen & Doreen Massey 
City Worlds [PDF ebook] 

For the first time in history, half of the worlds population is living in mega-cities. Never before have we confronted such a geography of the worlds people.Analysing cities through spatial understanding, City Worlds explores how different worlds within the city are brought into close proximity. The authors outline new ways to address the ambiguities of cities: their promise and potential, their problems and threats.