Having "stumbled" into web design almost ten years ago, with no real visual design background to speak of, I have over the course of time picked up principals. This was no easy task, and meant trawling countless websites and articles, being intimidated and awed by the breadth of knowledge and theory that is required to even suggest that you have an idea of what visual design is all about. Some of the articles I read required obscene amounts of concentration and application to the task at hand, as well as some difficult and surprising mental leaps.
A couple of years ago, "The Zen of CSS design" went some way to solidifying some of the pricipals I had learned, and helped guide me in new directions, yet still at times was a little inaccessible and while it is a great reference for themed ideas and principles, this new book has frankly blown me away...and I really wish it had been written a long time ago!
I possess several SitePoint books, but only two others have I read cover to cover in almost one sitting, and then revisited; Kevin Yanks's PHP book and Stuart Langridges Java/ECMA script and DOM book. Not only was the content of these books superb, but the writing style was infectious and consequently the ideas were absorbed quickly. The same is most definitely true of this publication.
Targetted largely at the coding/programming end of the market, it essentially provides the reader with a firm grounding in the ideas, theory and some history of visual design, breaking it down into sensible chunks and providing just the right level of information to leave you not only with a solid base, but thirsting for further knowledge.
This publication could have saved me quite literally weeks and months of stumbling research had I discovered it years ago, and even now is a brilliant refresher for those of us unfortunate enough to have pursued a "proper" degree ;).
For budding and established web designers, I cannot recommend this book highly enough.