Well over 15,000,000 books and magazine issues may be searched in Google Books. The numbers grow daily as Google moves forward with its mission of scanning literally all the world's books and magazines. Even materials that are not readable full text are nonetheless searchable. Some texts (pre-1923) may be read in their entirety; while others may be read across a limited number of pages (20% of the book) in 'preview' mode and some are only viewable in snippet view. The latter is the least useful to researchers.
This massive scanning effort is not without controversy in a variety of aspects; but technologically and legally the effort is remarkable. In addition to reading and downloading public domain books, and searching across the Google Books universe, many books may also be purchased to be read on a computer and a diversity of e-readers (excluding Kindle) http://books.google.com/ebooks. Samples are available. Books purchased and public domain books may be added to a user's personal library and are brought together in My Library. Organize your Google reading using this feature http://books.google.com/googlebooks/mylibrary/. You must sign in to use it https://www.google.com/accounts/Login?service=print&continue=http://books.google.com/books%3Fop%3Dlibrary&hl=en.
Serious users, as appropriate, should use the advanced search screen http://books.google.com/advanced_book_search. Texts may be read using the online reader or as a PDF. PDFs may be downloaded into software readers such as Adobe Digital Editions (ADE) http://www.adobe.com/products/digitaleditions/ and Adobe Reader http://get.adobe.com/reader/ as well as loaded on various e-readers. ADE and Sony support bookmarking and note taking. Load public domain EPUB Google books onto a Kindle using either Calibre http://calibre-ebook.com/ or RetroRead http://www.retroread.com/. Google books already in PDF may be loaded directly. If this does not work, email the Google Books’ PDF file to your free.kindle.com email address. Amazon will send the adjusted and now readable file back to you. Online readers may also read their Google Books using Google's free GooReader http://gooreader.com/ (a priced version enables the creations of PDFs). It allows one to read purchased Google Books as well as books freely available on Google. Since the full text of all material in Google Books has been scanned – even if it is not available to the reader – Google scanned texts may be mined. Text mining is made possible for subsets of Google Books with Google’s Ngram Viewer software, available at http://ngrams.googlelabs.com/. To stay current with Google Books developments read the blog Inside Google Books (ceased in 2012) http://booksearch.blogspot.com/. See also http://books.google.com/googlebooks/about.html. For the Google take on the Google Book Settlement see http://books.google.com/googlebooks/agreement/. For interpretation from the American Library Association see http://wo.ala.org/gbs/. For an overiview, try the Wikipedia entry Google Book Search Settlement Agreement http://en.wikipedia.org/wiki/Google_Book_Search_Settlement_Agreement
Reading on an Android, BlackBerry, iPhone, Nokia S60, and other Phones: For up-to-date information visit the Google Mobile Blog at http://googlemobile.blogspot.com/. There are over 1.5 million free texts to read. They've been scanned using Optical Character Recognition (OCR) so they will flow on a screen. Learn about this at: http://booksearch.blogspot.com/2009/02/15-million-books-in-your-pocket.html. To use this service go to: http://www.google.com/mobile/.
Google Books are increasingly (but perhaps not completely) present in Google Scholar http://libweb.lib.buffalo.edu/pdp/index.asp?ID=185. See the Google produced http://books.google.com/googlebooks/midterms.html and the University of Michigan's http://www.lib.umich.edu/files/services/mdp/GoogleBooks.pdf. The latter illustrated guide also explains how to find journal issues and articles. Google Books will help you trace the birth and longevity of an idea, assess the impact of a particular scholar, explore the evolution of language, verify quotes and discover the origin and use of quotes and passages across multiple texts, answer research and reference questions, create and search a rich corpus of primary source material (particularly from pre-1923 material). Note, full-text magazine material even after 1923 is abundant. What you find is limited by your linguistic creativity; subject headings for Google Books are generally insufficient and sometimes even incorrect. HathiTrust records offer better metadata http://libweb.lib.buffalo.edu/pdp/index.asp?ID=496. Any serious research project – especially in the humanities and social sciences – should include a Google Books search and a HathiTrust Digital Library search.
Books and magazines are restricted by neither language nor date of publication. Very old material is rare, however. Use the advanced search screen http://books.google.com/advanced_book_search to isolate material by either publication date or language, or other criteria.