The Principles of Beautiful Web Design is a not a PhotoShop tutorial or a set of CSS recipes, instead it's a primer on graphic design principals backed up with some on-line resources. If this sounds a bit like another recent SitePoint title, Sexy Web Design, then you'll probably want to know what the difference is that Sexy focuses more on the unability side of website design, things like: navigation, grouping elements, etc. while Beautiful talks about things like picking colours and typography; so really they compliment each other. Now if actually learn PhotoShop, both books are going to recommend the Photoshop Anthology, also from SitePoint.
The book itself is broken into five chapters: Layout and Composition, Colour, Texture, Typography, and Imagery. Each chapter breaks its topic down into a few sub-sections; for instance, the Colour chapter talks about the psychology of colour (with a bit of a western bias - some colours like yellow or white quite different associations in other cultures), theory, creating a palette, and ends with a application, in this case creating a palette for a website. The main project going through the book is a (fictional) tile store in Florida, Florida Tile, that pops up at points throughout the book to illustrate various concepts. To make that clear: the book doesn't take you through all the steps create the Florida Tile website, rather it is used to provide examples of things they are covered in the book's chapters. The book builds this website though the five chapters that make up the book.
Principles of Beautiful Web Design makes a good book to have along with the more practical books on the market today. Principals' covers Layout and Composition, Colour, Texture, Typography, and Imagery. There's a lot of books (and websites) that will teach you how to create 3D buttons or put together a "Web 2.0"-looking website, but when it comes down to building a website from scratch, a book like this is handy to have around.