I think it's a good unit of comparison. Virtually everyone has listened to a song to completion, and a very large number of people have done so in the past 24 hours, or even hour. It's also much easier to quantify the length of a song. Whereas with books: well, it's been a couple months at least since I've finished a new book. And number of pages in a book is not as meaningful to the human experience as amount of time. And the amount of time spent on a book -- i.e. minute per page -- greatly varies per human.

Sure, according to who you ask, a 3-minute Justin Bieber song contains less "data" than a 3-minute Bob Dylan song, but at least the quantifying of time is consistent among different people (um, relatively speaking).

And sure, 26 million songs is still as hard to comprehend as 26 million books. But again, more people can quantify how much of their life a song takes because most people have more recently consumed a song's worth of information.

The variance between data storage for song (e.g. length, kbps) is not meaningfully different enough in terms of order of magnitude.