First up are the content and site usability guidelines as put forth by the
engines themselves, that is, Google Webmaster Guidelines, Yahoo Search Content Quality Guidelines and MSN Guidelines for Successful Indexing. She provides links to all three.
There are also links to SEOmoz' and SEO Book's toolboxes, which include tools that determine the keyword density and overall SEO "strength" of a site, as well as keyword research tools and a Rank Checker. Batten also suggests tapping SEMPO's Learning Center, the SEO Chat forums, as well as Bruce Clay's Search Relationship Chart and Danny Sullivan's Intro to Search Engine Optimization primer (hosted by Search Engine Watch).