GoogleGuide, a comprehensive guide/tutorial to searching Google by Nancy Blachman
Google's own Help Central and Search Features pages
Nancy Pressman Levy's guide at Princeton, on which this is based.
See this guide if you're curious about the controversy surrounding the Google Books Project.
Peter Jacso's comprehensive review of Google Scholar. Jacso's own site has many more articles on searching and indexing.
This article from the Chronicle of Higher Education documents some of the problems with the metadata in Google Books.