Jakob Nielsen's Alertbox: Useit.com is the pulpit of Jakob Nielsen, who is widely considered to be the world authority on web usability. He's been writing a fortnightly column for his website since 1997. When perusing his nuggets of wisdom, start with the articles in bold.
Web Content Accessibility Guidelines from the W3C: although a very long read, this document provides a list of excellent accessibility recommendations arranged by priority.
Steve Krug has written an exquisite book on web usability: Don't Make Me Think! (ISBN 0789723107) Krug started out as a writer, and his style is funny and concise. He suggests humorous ways to test your site, too.
Jakob Nielsen, whose Alertbox column is mentioned above, has written several excellent books on web design. Although the examples in his books are usually dated before they hit the shelves, the advice in them is often timeless. They are:
Robin Williams wrote a nifty book called The Non-Designer's Design Book (ISBN 0321193857). I have the first edition, which predates web design. I've still applied oodles of advice about design to both paper and online documents. It's the kind of book you can absorb very quickly and refer to easily. I don't recommend her web book, though, which I found offered many suggestions that prioritized visual snazziness over practical usability.