First is the UCL section of today's journey. That being the Bartlett School of Architecture at the University College of London which IMO has the best minds working on cities and understanding them today this side of Chris Alexander (and with that said, include anything by Chris Alexander, an extensive catalog in itself):
Bill Hillier's: Space is the Machine. The foundation of Space Syntax and the culmination of Hillier's career effort towards finding ways to objectively measure and predict urban performance objectively free of the shackles of theory, subjectivity, and personal taste. There are rules to cities and some can't be broken no matter how great the urge against the repression of personal creativity.
Michael Batty's Cities and Complexity - Batty, in my understanding was the first to formally connect the dots between the Benoit Mandelbrot's and company and their mathematical study of complexity and fractals and with cities. Circular and logical in a way that the connection was made, as Mandelbrot et al were inspired by Jane Jacobs' writings about the city. The city is a fractal of oft-subjective desirability, of attraction and repulsion.
Stephen Marshall, like the two previous authors/scholars, is also at Bartlett. Much of the material seems remedial, sometimes tedious, but this is always necessary foundation for the more profound moments of both books. It, also like the works above, is highly referential of the other Bartlett boys without being an echo chamber. Each builds upon the other's work incrementally advancing this way of understanding cities, in the pattern of a fractal itself.
Lewis Mumford's The City in History. Past, present, and future explained through the consistent processes. Truly indispensible and a brick of a book, but worth it primarily for his writing of the donut city in the midst of the hollowing out process. Mumford has been somewhat of a marginalized Cassandra. How dare you pooh-pooh progress?! Oops. He was right.
Spiro Kostof's City Assembled and City Shaped series. Perhaps the most exhaustive walk through the history of cities, their morphology, and the context change occurred within.
Death and Life is held up as the modern bible of urbanism, but I prefer these two. Where Death and Life observed and described, Economy of Cities and Wealth of Nations explained.
Traffic by Tom Vanderbilt. Easily, EASILY the best book of the last decade on cities. Incredible depth and breadth achieved by a writer outside of the profession of cities, meaning it is a rather enjoyable read for its duration. Vanderbilt explores a variety of sciences to help explain traffic with particular attention to the psychology of driving.
Not a book. DVD series, and a life changing one. If you aren't terribly familiar with the hierarchy, dynamics, and emergent protocol of gang life, this might be the closest you'll ever get. Season 1. Wonder about the effects of the decay of the American industrial base? Season 2. Not sure about the political machinations and corrupting agents in city hall? Pop in season 3. Wonder why our schools are truly dreadful, in more ways than one, season 4. The Wire has street cred. It is branded as authentic. It hired people who actually lived many of the lives of its characters as actors. The creators/writers were a journalist covering the city and a cop turned teacher. And they added rather famous urban novelists to help construct the modern day greek tragedy of the American city that is just as much Dallas or Detroit as it is Baltimore. If any of us are alive in 2050, the Wire will still be relevant.
Beyond these that are all directly focused on the urban, I also recommend the following that can be tangentially related to city form and systems:
Critical Mass - Philip Ball (physics)
Collapse of Complex Societies - Joseph Tainter (anthropology)
Collapse and Guns, Germs, and Steel - Jared Diamond (anthropology)
Cradle to Cradle - Bill McDonough (sustainability)
Beyond Growth - Herman Daly (sustainability meets economics)
Genius of the Beast - Howard Bloom (capitalism meets biology and evolution)
Diversity of Life - E.O. Wilson (ecology/biology - cities are human ecologies)
Biomimicry - Janine Benyus (biology -> design)
Here Comes Everybody - Clay Shirky (power of networks and agglomeration in social media)