The Google Book Project has thousands of books of public domain volumes, where you’ll find quite a nice selection to choose from. With Google Book Downloader, you can save book as PDF from google to your local filesystem.

Eventhough it is easy to download books from Google Book Projecy, Google Book Downloader will not let you pirate books. It will however let you download books that are flagged as full-access such as books in the public domain and books with limited-preview where you’ll only get the preview parts, not the entire book.

To learn more about Google Book Downloader, checkout this tutorial.