All-you-can-eat subscriptions work well for digital magazines, music and videos; why shouldn't they work for e-books? Oyster certainly thinks they should, as it just launched a flat-rate book service for iPhone users. Members pay $10 per month for unfettered access to about 100,000 books from HarperCollins, Hougton Mifflin Harcourt and smaller publishers. The unlimited-use model also facilitates a Goodreads-like social component that shows what friends are reading at any given moment. Bookworms can request an invitation today. Oyster hasn't said if or when the service will go without invitations, but it hopes to release an iPad app in the near future.