A few books to consider when pondering the Social Web:
The Tipping Point by Malcolm Gladwell (2000). How little things can make a big difference. The second chapter discusses Stanley Milgram's famous 'six degrees of separation' that I relate to the Social Web. Gladwell's entire book, morever, encompasses the concept of memes (contagious ideas). "Memes", by the way, was coined by biologist Richard Dawkins in The Selfish Gene (1976)
The Wisdom of Crowds by James Surowiecki (2004). Using examples as far flung as the Who Wants to Be a Millionaire game show to finding a submarine on the ocean floor, Surowiecki demonstrates how the Many are usually smarter than the Few. To me this explains the success of wikis, Google, and the Internet itself. Heck, it explains economics [shades of Free to Choose by Milton Friedman (1980)] and a good chunk of politics, too. You can read my blog entry about it if you'd like.
Generation Me by Jean Twenge (2006). An extremely compelling look at the changing generations (Boomers to GenX to GenMe) and how each relates to the world. The author speaks in broader terms than something as simple as blogs, but you won't wonder why young people spill their hearts into blogs after reading this book. I wrote a blog about this one, too.