There is a wealth of information about search engine optimization (SEO) and related topics on the web. However, it is not all reliable as the landscape changes constantly as the search engines change their algorithms.
Practical Ecommerce has an SEO section that is invaluable. Their writing is very accessible.
Another place I recommend to learn is the Google Search forum at Webmaster World. If you really are interested in the topic, purchase the membership and read the Supporter's Forum. Some of the top names in the search engine field spend a lot of time on that forum (including representatives from Google).