SEO + site maps - study by Googlers

Get hold of a Google site map study, a 10-page PDF covering all you need to know for search engine optimisation (SEO). You can read a summary of the site map study report on search engine land blog.

It covers recent years of sitemaps useage at Google, working out how sitemap files can improve Google web index freshness and coverage (how Google crawls the web deeper to discover more content than if there was no sitemap).