The author or publisher set a price for giving the book to the world. Once you and your fellow ungluers raise enough money to meet that price, the Unglued ebook is available at no charge, for everyone, everywhere!
Are the unglued ebooks compatible with my reader?
Unglued ebooks are distributed with NO DRM, so they'll work on Kindle, iPad, Kobo, Mac, Windows, Linux... you get the idea. And if the ePub format isn't best for your device, you're free to shift unglued books to a format that works better for you.