A patent applied by Amazon in 2006 has been made public. Depending on the interpretation, Amazon's patent may be broad enough to justify a lawsuit over devices like "Nook and Alex," both of which sport a design clearly claimed by Amazon.
"5. A handheld electronic device comprising: a first display for presenting visible representations of content, the first display comprising an electronic paper display; and a second display positioned alongside the first display, wherein the second display includes a plurality of graphic elements that correspond to portions of the first display, and wherein the second display is responsive to user input to one of the graphic elements to perform at least one action on content shown in a portion of the first display that corresponds to the one graphic element."
More Info: Amazon Patent