Steven Johnson's Emergence: The Connected Lives of Ants, Brains, Cities, and Software (first chapter) is not a landmark or groundbreaking book (and it's not supposed to be), but it got me more excited about thinking about the world than any book in recent memory. Emergence is a good introduction to "complex adaptive systems that display emergent behavior" or, as Wired magazine puts it, "How Dumb Components Give Rise To Smart Systems". Highly recommended, if only for the bibliography.
Reviews of Emergence: NY Times, The Guardian, Wired magazine, Village Voice, LA Times, Chris Locke, Amazon
Related online material by Steven Johnson:
Play It As It Learns, The Swarm Next Time, Interview with Will Wright, Why Everything Is All That, Interview with Deborah Gordon, Interview with Mitch Resnick, Interview with Richard Rogers, Interview with Steven Pinker.