You can get the basics at: http://drupal.org/theme-guide, also search the web for Drupal Theming, you can find a lot of free blogs, posts, pod casts, screen casts, ect.
One really great site is: http://mustardseedmedia.com/podcast, I just found out that Bob is back, and it is better than ever.
On the pay for stuff, there are a bunch of books like: Front
End Drupal: Designing, Theming, Scripting
But my favorite is: www.lynda.com, they have 3 Drupal titles, the newest one being on Theming,
Hope that helps,