Bookbub IS great! You can subscribe and select what type of books you are interested in and they will email you a list of free and other books every day! It lets you download them from several places. I use a Kindle reader app on my tablet so I usually go to Amazon Kindle to get the free book. It stays on your device until you delete it, then stays in your Kindle library until you delete it.
I also use Overdrive to get books from the library, but I quit when I found all the great books available for free that don't expire in two weeks.