Brad Phillip's Media Training Bible is an invaluable resource for any communications professional--agency or client-side. I would even recommend this for executive directors of small nonprofits who don't have access to communications counsel.
Phillips covers all aspects of dealing with the media (including social media), from messaging to appearance. Through easy-to-read lessons, he provides practical, tested tips and information that will help anyone who has to give a presentation or speak to a reporter.
The section that I found the most helpful was Messages and Message Supports, where Phillips provides a useful approach to developing messaging that is relevant to your audience using CUBE A components (consistent, unburdened, brief, ear-worthy and audience-focused). There are many spokespeople who tend to go "off-message" or whose message is too complicated, who could benefit from this exercise.