This is very cool. My only critique would be that they restricted it to the iPad:
So it comes as rather a surprise to learn that Mr. Gass has written a 15,000-word essay called “Abstractions Arrive: Having Been There All the Time,” a collaboration with the noted photographer Michael Eastman, that can be read only on an iPad equipped with iBooks 2. Abstractions Arrive is published for $4.99 a copy by Stephen Schenkenberg, who runs the excellent Reading William Gass blog. (Every serious writer should have such an informative blog; few do.) It is not available in any other format.
And of course, you should check out my and Barrett Hathcock’s own entry into the ebook long essay, Lady Chatterley’s Brother.