The best book on social influence

I read a lot of business books, in fact I'm a bit of an addict but most turn out to be a great disappointment, full of regurgitated and hackneyed cliches but Return on Influence by Mark Schaefer is by far the best book I've read about influence marketing. It's clear, erudite and full of common sense and insight, and I've already bought copies for three of my clients (I wonder how that might get ranked as influence). I don't think you can really cast judgement on Klout until you have read this book; it certainly has changed some of my views.