GMAT Advanced Quant: 250+ Practice Problems & Bonus Online Resources (Manhattan Prep GMAT Strategy Guides) 2nd Edition, brand-new.
This was bought together with the full Manhattan GMAT strategic set as a plan B. It had been a while since my Uni days, so this was my insurance on Quant. It turned out that my work kept me too busy to use it, otherwise I may get an even higher score.
Anyway, this is a book full of sample questions and techniques for the "advanced" quant problems. I say it's a good read if you are devoted to be the best of the best.