Library is a project that is designed to read an ebook (or later, a series of ebooks) and create a UI that is easy to consume.
It is essentially a "web" implementation of eReaders, such as Google Books.
As part of ongoing work to create consensus around various operational topics there are a series of books being written. These books are designed to be sharable, especially while they’re in draft but later to a broader audience.
There was few implementations of eReaders around, and unpacking the files on disk did not yeild a satisfactory user experience.