For people learning CSS I can recommend Bulletproof web design: Improving flexibility and protecting against worst-case scenarios with XHTML and CSS
It’s very well explained and starts with an example of a site that does things in a ‘wrong/not optimal way’ (ex: using nested tables for layouts) and then explains how you can do it with css.
I read it a few months ago and now make quite complex css-designs without much trouble with IE.
I suppose that using css improves your google rankings, because you can easily put your most important content first in the code. But more importantly: making correct use of css improves usability and gives your visitors a better experience!