Kindle Digital Publishing announced a plan earlier this week called MatchBook that will allow purchasers of paperbacks/hard cover books to buy the digital version at a reduced rate. Authors must opt in to this program.
For a great explanation of how it works, see Cheryl Kaye Tardif’s post at the Indie Chicks CafĂ©.
Source:
http://www.marypathyland.com/thehylander/2013/09/05/kindle-matchbook-explained/