Theme sources

W3Schools' succinct but comprehensive interactive tutorial for learning CSS
W3C Learning CSS
WDG Guide to Cascading Style Sheets
CSS Discuss Wiki is a valuable collection of CSS tips and tricks. The information is collected from css-discuss.org discussions.
The growing link collection Web-dev-bookmarks with links about Creativity, CSS techniques and examples, Web design, Ajax, Applications, Usability etc
HTML Dog - HTML and CSS tutorials, references, articles and news
Digital Web Magazine book reviews on CSS and Web design