Books that are not in the public domain are the intellectual property of the author of the book (or to whomever has received rights from the author). DRM technologies are used to limit the amount and type of access and control a consumer has over the book after purchasing it. For instance distributors of information may limit the number of pages that can be downloaded at any one time, limit the ability of the consumer to share the book, or make the book 'vanish' once a loan period is completed.
Adobe Digital Editions
The primary tool for managing DRM on your computer is Adobe Digital Editions. To use Adobe Digital Editions on a computer:
Current Apple and Windows operating systems support Adobe Digital Editions DRM.
Mobile Device Downloads
To download DRM protected books to a tablet device such as an IPad you will need to use a combination of Adobe Digital Editions and an ereading app, such as Bluefire Reader that is equipped to manage digital rights (view a list of readers that support DRM, i.e. that are"library compatible").