My favorite book on writing comes from advertising. Two authors, Kenneth Roman (former Ogivly CEO) and Joel Raphaelson (former Ogivly creative director): Writing That Works (http://www.amazon.com/Writing-Works-Communicate-Effectively-...)

This book has been transforming my long and vague e-mails, letters and blog posts into more crisp and to the point while keeping the tone of the message. Still learning, the book has tons of concrete advice to follow.