Google Books features the full text of more than 40 million books in addition to select magazines and newspapers. There is full text for books in the public domain (pre-1927) and limited previews of copyrighted materials. Google Books particularly excels in primary source and public domain images for historical and genealogy researchers. 

Use the main search to navigate the holdings. You can limit your results by language (46 languages recognized) and date range, and there are many options for searching such as by title, author, publisher, subject, and more. Each result has a "search in this book" box. There are helpful tools such as zoom, embed, "buy this book," and "find in a library." 

Alternatives to Google Books are the Internet Archive and HathiTrust, both of which have digitized millions of books. However, they are not as large as Google Books.