Ink screen e-books usually required the purchase of corresponding reading resources to use. Some e-books that were free to read might use advertisements or other forms of free resources, but they were not necessarily completely free. For example, Amazon's e-books for the Kindle-brand could be provided through free books and a service of a single purchase, but the Kindle-brand also provided advertising and membership services to provide additional revenue. Other e-book applications might offer free books but require the application's services for a purchase or advertisement. If you want to read e-books for free, you can find some open source e-book libraries such as EPUB and MOBI-format. These files can be read on a variety of devices, including ink-screen e-paper books.