"Diffusion of Innovations"
by Everett M. Rogers
Link: http://www.amazon.co.uk/Diffusion-Innovations-Everett-M-Rogers/dp/0743222091/
Reading history and reviews
Finished in 2006
Someone at work recommended this, it's essentially a text book on diffusion theory, but it's surprising accessible. Some of the examples are really fascinating, and it totally kicks the ass of Malcom Gladwell's Tipping Point book. (It did take a lot longer to read though.)