You Can Negotiate Anything is a classic, and very inexpensive investment considering the upside.
I can't give you any advice that you won't find in this book.
Good luck.
For longer term and more general help with persuasion tactics (which might be used to get you to agree to a lower offer), I recommend Influence: The Psychology of Persuasion by Robert Cialdini: http://www.amazon.com/Influence-Psychology-Persuasion-Busine...
You will easily recover the cost of these two books using the knowledge from them.
I can't recommend You Can Negotiate Anything by Herb Cohen enough!
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