Great article from Sitepoint on how to code HTML newsletters. Definitely worth reading. They even linked to some additional resources at the end of the article. Long story short: Keep things simple. A single column table layout works best (and degrades the most gracefully) in the most email clients. CSS is allowed but stripped out by many clients so be conservative with its use.

Sitepoint is a great resource. Great tutorials, great articles, and a great blog section.