While Google should not be your only means for searching for sources, it is an essential search tool and can be made more efficient by using some of the filters and specialised Google search engines like Google Scholar and Google Books.
Google Books is made up of costed and free resources mostly from publishers and libraries. You can download PDF files of entire books if they are out of copyright or if the publisher has given permission. Also included in Google Books are Worldcat links which can be useful in finding the physical location of books around the world.
Tips for Google Books: