The authors bring out the merits of the principled negotiations over the traditional hard/soft negotiation techniques. The book recommends to focus on the people, their interests, available options and the standards in any negotiation. I liked the BATNA (Best Alternative To Negotiated Agreement) concept which can be applied to many real life negotiation situations.
The content is short & sweet. This book is focussed on principled negotiations and doesn't deal with all the negotiation paradigms. I would consider this as a primer for negotiations. At the end, there are a few techniques suggested to handle people who play dirty tricks.
You cannot learn to swim by reading a book. Similarly, you cannot be a negotiation expert overnight by just reading this book. You should perpetually apply the ideas.
Author's website: http://www.williamury.com/