Google Books - https://books.google.com.au/
Google books is a searchable comprehensive index of full text books, some of which can be retrieved in full text. However, it also includes commercial texts which it may retrieve and allow access to just snippets of, or sometimes certain chapters.
The functionality enables you to see which books may cover certain keywords or topics - so it can be a useful search mechanism to search full text to identify useful books, which can then be accessed elsewhere.
You can then see information about the book, which can then assist you in locating the full text book if needed.
Some older works that are in the public domain have been scanned and can be viewed and downloaded off the site in full text
Uses a standard google search to search the full text of millions of books, some available in full text
Sample of results. You can filter results by various options.