Barnes & Noble will no longer carry books published by Amazon in its stores, according to the Wall Street Journal.  Amazon has been expanding its publishing efforts, both directly with authors and by purchasing a line of children’s books.  Amazon had recently announced a deal with Houghton Mifflin Harcourt to sell its physical books to the book trade.  But they will now not be sold in Barnes & Noble’s stores, although B&N will continue to carry Amazon titles on its Website.
This move is actually based on the same principle first acted upon by Barnes & Noble when it dropped 100 DC graphic novels from its stores after DC made an exclusive deal with Amazon for those titles in digital form (see “B&N to DC—It’s On”): Barnes & Noble will not offer products in its stores that are not available to it in all forms in which they’re sold.  That means that if a publisher (including, in this case Amazon) does not sell B&N digital books when it makes those books available to other digital retailers, the books come out of B&N’s stores. 
The vertical integration of retailers into publishing has been shifting rapidly recently, with Amazon moving upstream aggressively, while B&N is selling its publishing arm.