All very good suggestions -- though all focused on just one form of urbanism.

If I had to recommend just one book on planning it would be Cities of Tomorrow by Sir Peter Hall. All his books are wonderful, his knighting is no accident. This one is chalked full of original chapters.

Although not a monograph, The City Reader is an exceptional collection of most of the seminal works on urbanism across sociology, psychology, geography and planning. It is probably the most commonly used reader across urban geography.