I agree that there's loads of free resources out there.
But as far as books do go though, the only one I ever recommend really seems to be "Designing CSS Webpages"
http://www.amazon.co.uk/exec/obidos/ASIN/0735712638/
It might be that it was just the perfect book for me at that time, but after reading it I did think I'd figured out what all this new fangled (at the time) seperating content from presentation meant. It might be getting a bit old now though.
A while ago I also said I didn't see the point in buying Dan Cederholm's Web Standards Solutions: The Markup & Style Handbook, as everything's already been covered on his website. Well someone bought it for me a few weeks ago and too be honest, I'd recommend it now.
I like books I can have at the ready on my desk but aren't just full of appendixes are syntax and can actually be read away from the computer. And it's one of them.
http://www.amazon.co.uk/exec/obidos/ASIN/1590593812/