Jeffrey Veen, an established design guru and one of the creators of HotWired.com, has authored with The Art and Science of Web Design
a carefully structured look into the undercurrents of Web design. Organized around the key development topics, the book is laden with a historical background of standards, features, and trends. Yet the topics are timeless and core to good Web engineering, so it's space well spent. The mix of expert opinion and historical explanation creates a well-rounded reader experience.
You can now download for free the full book (in PDF format) with illustrations from Veen's website