If your HTML/XHTML and CSS is still not standards compliant and are still struggling to decide whether it’s worth it or not then “Designing with Web Standards” by Jeffrey Zeldman is an essential reading.
This book was recommended by my professor at NYU when I took the class on “Web Development with CSS and XHTML”. Finally, I found time to read this book. Since I have been using XHTML since about one year and a half, I found myself skipping through most of the content. The most useful part for me in this book was the resources on accessibility. I read that part very carefully and is now a must in all websites I code. So, it was worth reading this book.