This book provides a good basic overview on how Magento Themes work and how to go about creating your first theme. It provides a clear, logical breakdown of how Magento works with Layout, Templates and Stylesheets, better than the designer guide on the Magento connect website.
The only problem is that, while it starts out at almost too basic a level (telling you exactly where to type things, which button to press etc.) it then suddenly jumps to rewriting complex .phtml and .xml files, assuming quite an advanced knowledge of PHP if you really want to do anything yourself - hopefully this gap will be addressed in the next update as Magento guides will always be trying to hit a moving target.
I would recommend this book if you are trying to get to grips with Magento for the first time and already have some grasp of PHP (as otherwise what you will be able to achieve will be rather limited).