It's amazing to me that this book was written over twenty years ago, but is still so relevant. Negotiation is a passion of mine, and I have read this book multiple times because the ideas presented in it are the basis for almost any book that has been written on negotiation since its publication. Plus, it is a quick read that almost anyone can understand.
This book revolutionized negotiation with its claim that you would be better off if the person that you were negotiating with also read this book. Rather than focusing on tricks and ways to manipulate the other side, it shows you how to set up a cooperative, win-win negotiation.
Such terms as win-win negotiation, cooperative problem solving, BATNA (best alternative to a negotiated agreement), and negotiation jujitsu might sound trite because they are used so frequently in other negotiation texts. However, I'm willing to look past that since these terms originated here.
In multiple negotiations--big and small--I have used the process outlined in this book (1. "separate the people from the problem", 2. "focus on interests, not positions", 3. "invent options for mutual gain", 4. "insist on using objective criteria") to produce successful results.
Your ability to negotiate affects so many parts of your life (from how much money you make to how you resolve conflicts with your spouse) that it is worth investing in this book and in becoming a better negotiator.
While (because the book is a tad idealistic) I do not recommend making this the only book that you read on negotiation, I highly recommend it as one of the books to read. I'd also recommend checking out "Getting Past No" by Bill Ury, which is the follow-on to this and discusses how to handle situations in which the other side doesn't want to cooperate.