I was just asked what was the most interesting tech-related book I've read in the last year and I had to say it was the 5th edition of Elizabeth Castro's Visual QuickStart Guide to HTML. It might not seem inspiring in and of itself, but it's so good I have a copy at home and at work always within arm's reach of my keyboard. It's brilliantly designed (the format of the Visual Quickstart Guides is excellent), extensively indexed, succinct, readable, reliable about warning of browser quirks and incompatibilities, and best of all, so far as I have seen in the 5th edition, error-free.
Sometimes simple usability and reliability is better than all the flashy stuff in the world, especially when it makes it easy for you to understand and apply the really huge potential of XHTML and CSS. I recommend it to anyone who writes HTML, particularly those making the transition from HTML 4.0 to XHTML and those learning CSS. What's best about it? You can pick it up, get the answer you need and get on with your web design fast. If you're getting a reference book everything rests on your ability to find the answer and quickly undestand the answer so you can apply it. This book is a stellar example of just that principle.
For those just learning HTML and CSS you may wish to also use another more wordy guide to get you started, but I suspect this is the one you'll still be using (and continually buying the new edition of) years down the road.