I’ve read the entire Web Style Guide 3 and love it.  It is essential to solid web design, as well as planning and production phases in web development.  As a relatively new medium, there are not yet time-tested conventions and standards for web design and development, compared to print which has been around for 500 years.  The Guide attempts to standardize web style based on traditional print conventions, user interaction research and web design trends that have become de facto standards in the last 20 years.  The Guide can be purchased on Amazon but is also freely available in an online edition.