"Book Search works just like web search. Try a search on Google Books or on Google.com. When we find a book with content that contains a match for your search terms, we'll link to it in your search results. If the book is out of copyright, or the publisher has given us permission, you'll be able to see a preview of the book, and in some cases the entire text. If it's in the public domain, you're free to download a PDF copy. If you find a book you like, click on the "Buy this book" and "Borrow this book" links to see where you can buy or borrow the print book. You can now also buy the eBook from the Google Play Store." (from their website)
Google Books is where you can look at a preview of a book or read the whole thing (if it is in the public domain). In order to download the free books, you have to set up a Google account, and it will be moved into a "My Books" account in the Google Play Store. You can also purchase eBooks (the price will be in red on the left side above the book's information, and then you will need to put in your payment information, such as a credit card).
Free eBooks that require you to set up an account with a credit card
You will have to do a search for the free eBooks available at these sites.
Amazon Kindle eBooks