The Definitive Book of Body Language by Allan and Barabara Pease.
I've always had an interest in psychology and the inner workings of the mind. In college I was fascinated by a number of books on our internal and external motivations. To be honest, I think understanding those things has helped me in business.
Recently, I was discussing the successes and failures of corporate meetings with a few small business owners and one of them recommended this book to me. The last book on body language I had read was in college so I picked up this one — published in 2004 — for a refresher.
Allan Pease is the world's foremost expert on body language and that becomes very clear in the book. The Peases cover everything from the basics to advanced concepts, including cultural differences and gender differences. The book has a general focus, ranging across applications that include personal settings (like at a party or when you're interested in someone romantically) as well as corporate settings that include meetings, office politics, and interviews.
Admittedly, I would have liked a book that went even more in depth on the business side of the body language equation, but this very comprehensive book does a great job of talking about many aspects of body language that an be applied to business: this includes building rapport, as well as diffusing difficult situations, and making the customer feel important.
If you interact with people — socially or professionally — this is a good read. If you want to connect with your customers and make them feel great about doing business with you, this is a must-read.