Here are some popular digital reading sources:

1. Amazon Kindle: An e-reader that allows you to purchase, download, and read digital books and other content.
2. Project Gutenberg: A digital library that offers over 60,000 free e-books that can be downloaded in a variety of formats.
3. Google Books: An online library that allows you to search for and preview books, as well as purchase and download digital books.
4. iBooks: An e-book application for Apple devices that allows you to purchase and download books from the iBooks Store.
5. Goodreads: A social network for book lovers that allows you to find and discuss books, as well as track your reading progress.
6. OverDrive: A digital library platform that allows you to borrow and read e-books and audiobooks from your local library.
7. Scribd: A digital library that offers access to millions of e-books, audiobooks, and other content for a monthly subscription fee.
8. Feedbooks: A digital bookstore that offers a wide selection of e-books and audiobooks, as well as the ability to upload and share your own content.
9. Project MUSE: A digital library of humanities and social science journals that offers access to scholarly research and peer-reviewed articles.
10. Open Library: A digital library that offers access to over 1.7 million free e-books, as well as a vast collection of other digital content.