From MakeUseOf Find Beautiful Free eBooks In Google’s New Shop [US Only]:
Finding Free Books

For those of us in America, though, Google’s book store appears to be just another book store at first glance. There’s the usual collection of New York Times Bestsellers, and many books for sale.
Keep scrolling down, however, and you’ll see this section:

This is the paydirt. Here you can get access to Google’s 3 million public domain titles. You can browse by simply going through the free titles here, or you can search for any work that is in the public domain. Any book without a price beneath it, you can assume, is free.
Once you find a book you like you can read the book online. Google offers a simple but very useful web-based ebook reader:

This elegant interface gives you a great way to read these public domain books, particularly if you set your browser for full-screen. The arrow keys behave as you’d expect them to, turning the pages, and you can quickly access the table on contents or search the text. Best of all, it’s tied to your Google account, meaning you can switch computers and keep reading from where you left off, without having to install anything…”