Along with Difficult Conversations and Getting to Yes this is one of three texts, plus handouts, used at a Negotiation course at Harvard Law taken by students all over the university--and by people from all over the world. At the end of the course, the students spontaneously rose to give the teachers a standing ovation. It's a very popular and valuable course--and this book deals with some of the techniques at the heart of it.
A lot of the principles discussed here is not just for lawyers or diplomats or labor leaders. It applies to any of those kinds of situations where you have to make a deal, get something from someone without getting taken. That might mean negotiating a raise, coming to a price, resolving a dispute with a neighbor.
It talks about such techniques and strategies as focusing on interests, inventing options--and knowing your BATNA. (Best Alternative to a Negotiated Agreement. In other words, know when and at what point to walk away rather than let yourself get pressured into something you can't live with.) It shares that common ground with Getting to Yes. That book though is more geared towards the general reader. Beyond Winning is longer, more technical, and much more geared to the lawyer.