There are so many things you can do from here that you may very well be overwhelmed. You can look into some client side scripting langauges or maybe some server side stuff, but if it were up to me, I would stick to [X]HTML and CSS and learn those two as best as you can.
As ronald said, try making a site without tables. I have never read that book you mentioned before (I have never read a book on HTML) but it may have advised you to use tables for layout. I could go into a whole speil about not using tables but even I am tired of the debate. Just know that tables aren't for layout, they are for tabular data. Try looking into places like
http://www.glish.com/css or
http://www.meyerweb.com/eric/css/ for a ton of information on css based layouts without tables.
I don't think there is ever a point in my life where I will feel like I have gained enough knowledge in XHTML or CSS. Everyday I learn something new. Heck, I just started into something other than HTML (php) about a month ago, more than a year after I started learning HTML. It's a constant learning process.
Try thinking up things that you would like to do that would stretch your knowledge. That is the best way to learn and improve yourself. That is the way I have learned everything I know.
In final, I shall give you a few sites that I found very helpful in my search for excellence
:
http://www.w3schools.com/
http://www.w3.org
http://www.glish.com/css
http://www.webmonkey.com
Good luck, and come back here to learn as much as you would ever want to
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