These 4 reading apps are joining a growing network of over 90 retailers, self-pub services, app developers, and publishing companies that have all elected to help each of their customers combine a customer's ebook purchases into a single location. The resulting personal libraries are accessible from multiple reading apps, including Readmill's own apps for Android and iOS. Readmill's partners are a diverse bunch of companies including publishers like Penguin UK, ebookstores like Feedbooks, metrics apps like ReadMap, and even self-pub services like Tomely.
The 4 reading apps making the news today include Marvin, one of the leading reading apps for iOS, The Floating Book, which offers an "adjusted" reading experience that is designed to reduce problems with motion sickness, Librarus, a platform agnostic reading app for your web browser, and Bookinist, a DRM-free reading app for OSX.
So far as I know these 4 reading apps represent the first partners who aren't joining as sources of ebooks (whether DRMed or DRM-free) or as apps that enhance/support your (paper or digital) reading experience, and thus they represent a shift in Readmill's plans. Up until yesterday Readmill could be viewed as the end point of a network of suppliers, but today they have become a major node in an indie ebook network.
It's also a pretty nifty way for the reading apps to add support for shared cloud storage, isn't it?