Typically, Google Book Search has not been high on my list of sites that I regularly use to search for content. However, recently I have had very good luck with searching this site. Over at Learning to Change Business, a blog that I write on executive education, you will notice that I have been doing research on the Mars Pathfinder mission. Google books actually produced excellent results for this query and I found two or three books on the subject that had not turned up in other sites.
Today's post at Google Operating System entitled Quantity Over Quality at Google Book Search explains a bit more about the status of the product and some of the problems associated with producing the site. I have not found a downloadable book yet, but the citations have been helpful.