The first prize is awarded to O'Reilly & Associates' Excel Hacks: 100 Industrial-Strength Tips and Tools.
This particular Hacks book has an advantage that many other Hacks titles don't have. The subject matter relates to a single program, so nearly all the hacks are applicable to an Excel user. Contrast that to a Hacks title like Network Security Hacks, where your choice of operating system limits the number of tips that you can use. While there are some Windows-only Excel hacks included, most all the tricks work on both the Windows and Mac version of Excel. Nobody should feel left out here.
Amazon.com : Best Books of 2004