One of the clearest explanations of Granovetter's theory appears in Malcolm Gladwell's book, The Tipping Point (pp. 53-54). This is not surprising since Gladwell has a gift for communicating academic research in a manner accessible to the wider public. All of his books (i.e., The Tipping Point, Outliers, Blink, and David and Goliath) include lucid explanations of research (whether it appeared in books or academic journals) that at first glance are impenetrable for non-experts.
Gladwell is featured in a recent Freakonomics podcast ("Being Malcolm Gladwell") that is wonderful. It focuses primarily on his book, Outliers, but regardless of whether you've read the book or not, it is well worth a listen. You can listen to the podcast at the Freakonomics website ("Being Malcolm Gladwell") or download it from iTunes.