This article [0] seems to put B&N and Apple around 20% each earlier this year pitting for no. 2 - with Apple (and Amazon) gaining against B&N. While Apple have decent market share (that's probably roughly comparable to iOS smartphone share worldwide?) and continue to sell a zillion iPads probably with iBooks installed and certainly with iBooks the only app you can buy ebooks in Amazon are probably pretty protected at a guess.
[0] http://www.digitalbookworld.com/2014/is-apple-now-the-no-2-e...