I have read many books about Magento and in my opinion Packt Publishings are some of the best. Although there are some good general and cookbook style Magento books around I think Magento Theming is an area that requires a dedicated book and I would say that if you are serious about your ecommerce store and getting the most from Magento I think Magento Theme Design 1.4 by Richard Carter is well worth a read - cover to cover.
It is also important to note that many of the books currently available for Magento are based on version 1.3 that this book is currently as up to date as you can get (as far as I am aware all of the book is relevant to the recently released version 1.5) being based on 1.4, this is especially important as there has been some significant changes in the area of themes in version 1.4 from 1.3.
There is no doubt that Magento theme designing can be daunting and the more knowledge you have of HTML, CSS, XML and possibly of other CMS themes the less steep the learning curve well be. There is also certainly a big incentive to change the default theme as to me it is very ugly. The results you can get from just some of the techniques in this book can be make a huge difference to the look of you Magento store.
Overall I enjoyed the style of the book and particularly found the build up of Richard's example site useful. A great feature of the book is that you can download the code used in the books which includes css files, xml files and even the images used in the examples.
If you have no CSS skills at all probably best to start with a good CSS book. Overall a great book for anyone looking to improve their Magento theming skills.