As a web developer, my first web design book was Build Your Own Website The Right Way... by Sitepoint. I thought it a good idea to be the first book I recommended, simply because if you're just starting out in web development, its really basic, but still teaches you a lot you need to know, and can apply later on.

I suppose the main reason why I recommend this book to every one thats just starting out, is that it makes it really easy to get up and running. You could go through the entire book in a weekend, and then be ready to start churning out your own sites straight away.

If you haven't got it yet, you should definitely check it out.

Build Your Own Website The Right Way Using HTML & CSS 2nd Edition