The obvious first place to go is www.w3.org/Style/CSS. It lists known books about CSS, online resources, supporting browsers, and editors.

The CSS1 Recommendation is at http://www.w3.org/TR/REC-CSS1

The CSS2 Recommendation is at http://www.w3.org/TR/REC-CSS2/

XHTML is at http://www.w3.org/TR/xhtml1/ with more information at http://www.w3.org/Markup

XML is at http://www.w3.org/TR/REC-xml with more information at http://www.w3.org/XML/

http://jigsaw.w3.org/css-validator/ will validate your CSS.

http://validator.w3.org/ will validate your HTML and XHTML

http://www.xml.com/xml/pub/tools/ruwf/check.html will check your XML for well-formedness.

http://www.w3.org/People/Raggett/tidy/ will tidy up your HTML, making it more amenable for CSS, and convert it to XHTML 1.0

http://www.w3.org/Style/CSS/Test/ lets you test your browser for CSS compliance

http://www.w3.org/StyleSheets/Core/ is a set of style sheets for HTML designed by a graphic-designer

http://webreview.com/pub/guides/style/style.html keeps track feature by feature of how well (or badly) browsers implement CSS.

http://webreview.com/wr/pub/guides/style/lboard.html shows which browser is doing best overall.

http://css.nu/pointers/bugs.html offers workarounds for browser bugs.