This website has been quite helpful as far as CSS reference:
http://www.csstutorial.net/

w3schools.com has been an indespensible resource for everything html/css 
etc

This has been a great resource for getting images for my project on the 
Bayeux Tapestry:
http://hastings1066.com/

A useful deconstruction of many of the elements of the Bayeux Tapestry 
provided by the BBC:
http://www.bbc.co.uk/history/british/normans/bayeux_tapestry_gallery.shtml

XHTML validator provided by w3.org - 
http://validator.w3.org/

Another resource that frequently came up in my search for help on HMTL and 
CSS was http://htmldog.com/

Whenever I am working in programming languages and I come across an 
obstacle or a concept with which I am unfamiliar, it is now second nature 
for me to 'Ctrl-Tab' into my web browser and 'Ctrl-t' to create a new tab 
and then immediately type in my issue.  I have never encountered an 
instance where my problem or my question is unique and I have always found 
the results to be more than satisfactory and a lot quicker than digging 
through the documentation.  I also found this true with xhtml and css.  The 
book was useful for reading and familiarizing myself with topics but while 
I was working on projects it was much more efficient to use Google to find 
solutions than to look through an index.  As such, I feel like I should 
give Google mention here!
www.google.com

Also, this has been a great resource showing examples of CSS usage:
artypapers.com/csshelppile/

Great for CSS tutorials:
www.echoecho.com/css.htm