I built my first simple web page at SAIC in 1992 when my good friend Gregg Hanna set up a publicly accessible web server for my working group. Since then I have had a lot of people suggest that my web sites could look better but frankly I have always been more interested in content and developing cool web application functionality.
Recently I have been putting some effort into improving my design skills and the best resource that I have found is "The Non-Designer's Design Book" by Robin Williams
The author Robin Williams does a fantastic job at explaining four basic concepts of design: contrast, repetition, alignment, and proximity. She then provides good examples that show the reader how to recognize bad design and how to correct design errors.
I spent some time redesigning my main web site and really enjoyed the process. I started by determining the worst aspects of the old design based on Robin's advice and then tried to correct the design flaws using her examples.
Ethan's short book on responsive design really helped me a lot because he efficiently covered what I needed to know about media queries and effective CSS and HTML5.
I have been using Dojo Mobile and more recently Twitter Bootstrap which did a lot of the heaving lifting for me in my first attempts at creating responsive multiple platform web applications. Reading Ethan's book helped me understand some of what Dojo and Boostrap were doing for me "behind the scenes" and also gives me some confidence in writing one page web applications from scratch without frameworks that might do more than I want and add unnecessary complexity.