Oyster is a new subscription service that lets members read unlimited books on their mobile device for a flat monthly fee. Members can choose from a growing library of more than 100,000 titles. The service is designed specifically for mobile (currently iPhone and iPod touch only) and includes social features that help members discover new books and recommend favorites to their friends. For more info check out this Oyster blog post. You can request an invite for Oyster on their site.
Oyster, New Service Offers Unlimited Books to Mobile Users for Flat Monthly Fee