This is not new; there is plenty of sheet music on the iBooks Store, but I had not noticed it before. The iBooks Store could be a great place to sell sheet music; you can use it on an iPad, and, with a retina device, this could be practical.
It’s interesting, however, that Apple is highlighting this specific score by Philip Glass. Given that this is contemporary music, it can be sold “full price,” unlike classical music which is in the public domain. I wonder if Apple is planning to expand the sale of sheet music to popular music as well. I’m not sure how big a market there is for books of sheet music of, say, Coldplay and Taylor Swift, but these books do sell, so why not sell them on the iBooks Store?