This is easily the best book I've read about speechwriting. It's such an enjoyable read and there's so much information - it's a proper bible for anyone who wants to sharpen their rhetorical skills. And it's all set out in a nice clear, logical structure, so you can quickly find anything you might want to refer back to.
Simon Lancaster has tons of experience writing speeches for senior politicians, celebrities and captains of industry. Whatever your own level of expertise you'll find plenty of useful, and often funny, insights here.
All the tricks of the trade are explained (of course) and illustrated with lively examples from speeches ancient and modern. But what sets it apart from so many other guides is the acknowledgement that a successful speech often depends as much on the quality of relationships - between writer, speaker and audience - and the clarity of purpose, as it does on persuasive techniques.
For this reason I'd recommend 'Speechwriting' to speakers as well as speechwriters - and indeed to anyone with an interest in the psychology of communicating to a wide audience.