Bookbub.com is a website that tells you about free or inexpensive digital books – eBooks.
You can read these books on an eBook reader (such as Kindle or Nook). But you can also read them on your phone, tablet, or computer – usually with the free Kindle App or Nook App. If you have an iPhone or iPad, you can read many of these using your iBooks app.
For example, on BookBub they have listed an eBook version of the popular children’s book Goodnight Goodnight Construction Site, selling for $1.99 on Amazon and Barnes & Noble.
And here is their latest listing of Free eBooks.