Project Gutenberg offers you over 40,000 free ebooks. You are free to download them to your device or you can read them online anonymously. They don’t charge any registration fees and this is the only website which Richard Matthew Stallman supports.
Recently Project Gutenberg added a new feature which you can use for downloading eBooks directly to you dropbox. Its easy to use, next time when you visit Gutenberg.Org you have to select the book by clicking it as usual but now you will find one more button added on the book’s table which allows you to download that certain book to dropbox. Its more quick and easy way of saving your favorite books to dropbox.