1. How to SEO Your Website – Good introductory article to those who know little about SEO.
2. SEObrowser.com – This is how your site looks to search engines.
3. Google’s Webmaster Guidelines – Google is as synonymous to search as Kleenex is to facial tissue. You’d better read their guidelines.
4. Robots.txt – Part of SEO is letting the search engines know what not to index. You don’t want them crawling around in every directory you have do you?
5. The SEO Theory White Papers – An academic approach to SEO.

In short – produce web sites that adhere to web standards, provide good desirable content, and don’t try to “trick” anybody into visiting your site.