Using HTML and CSS in a correct way, validated, and semantically and structurally correct, has a lot of benefits over ‘old style html’: faster and smaller pages, better results in search engines, easier to maintain, beter cross browser support, improved reusability, and better accessibility. Enough reason to start writing valid html code and use CSS for presentation.
One book which really helps you use XHTML and CSS is Bulletproof Web Design. Kathy Sierra blogs that another good book is now available: Head First HTML/CSS finally ships! I haven’t read the Head First HTML/CSS yet, but i do have three other books in this series: Head First Java, Head First Design Patterns and Head First Servlets and JSP. These are all excellent books, so this new books on HTML and CSS is probably also going to usefull.