Most of what I know about web design and development I’ve learned from New Riders books. I’m finally reading Don’t Make Me Think by Steve Krug, a book on web usability that practices what it preaches. (Those of you who have read the 1st edition might want to get hold of the new edition, which includes 3 new chapters.)
Other books on my shelf:
Designing with Web Standards by Jeffery Zeldman, just out in a 2nd edition
Eric Meyer on CSS by Eric Meyer, of course
Building Accessible Websites by Joe Clark
There are many more titles available on topics from Photoshop to writing for the web to search engine optimization.
I should also give a nod to A List Apart (the other ALA), where these authors and many more contribute useful articles that are free of charge.