I have had the good fortune to be Technical Editor for two books just published by Wiley. I recommend both of them.
The first is 'Excel 2007 PivotTables & PivotCharts' by Peter G Aitken. This is a good introduction to Pivot Tables for those who have not used them before. However, it is equally suited to power users who wish to increase their knowledge of Pivot Tables and how to use them more effectively.
The second is 'Excel & Access Integration' by Michael Alexander and Geoffrey Clark. This is aimed at the power user who wishes to start using Access and Excel together to leverage the unique capabilities of both these programs to build powerful office applications.