Probably the most useful since it goes into the psychology and strategy of problem solve more than other books.

Loren C. Larson Problem-Solving through Problems

Short and concise

Arthur Engel - Problem-Solving Strategies

Also one good link

http://www.kalva.demon.co.uk/

For preparing for quantitative interviews, the thing that you need to have down cold are problems dealing with probablility and statistics. Make sure that you know how to handle infinite series - specfically the telescoping methods.