Excel Books
Many people like to sit back and relax with a good book. There are hundreds of Excel books available, ranging from entertaining beginners guides (e.g., the "For Dummies" series from Wiley) to advanced books for developers.
I've written many Excel books, for users of all levels:
Or, you may find this list helpful:
Excel Resources
This section contains a variety of information about Excel, sources for help and selected Excel-related links.