Portable devices (such as Androids) are getting increasingly viable as ways to store and read books. A large mass of literature is available freely via projects such as Project Gutenberg.
Notably attractive on the desktop to help manage a whole "library" of electronic documents.
A framework for document structuring, publishing and search.
Allows generating documents using minimal markup (unlike XML!), and transforming that into a number of output forms suitable for publishing including the popular ePub format.