In a nutshell: READU reads txt, epub, and fb2 books you own, and tens of thousands of free books.
Other good stuff like:
- Download books from SkyDrive, Dropbox, Feedbooks, Gutenberg, and any websites.
- Translate and speak texts
- Pin a book to your start screen
- Live tiles with the new logo!
- Unique Read-to-sleep feature
- Passcode to protect your privacy
- Change of wallpaper