While I was on vacation, I had a few articles on my TabletPC that I'd like to have read. However, I couldn't bring myself to fire it up; its associations with work were so strong that it would've broken the holiday spell.
Every (paper) book I read has a different heft and feel; the covers are different, the pages have different textures, and the associations with each are distinct.
I remember where I read something, not only by where it was on the page and the spread, but how far through the book it was.
Fashion magazines are heavy, glossy, and smell of perfume samples; tabloids are light, fuzzy and disposable.
With music, I don't care how the information is carried; CD, radio, MP3, WMV, ... It doesn't matter. The associations are in what I hear, not what I see or touch.
This is a deep reason why the adoption of digital music has been so much faster than that of electronic books. Reading is a visual experience, and what you see matters. It's also a tactile one: even an e-book has to be held. Profound though subtle connotations are carried by the non-textual packaging. You can close your eyes and listen to the music; how it arrives at your ears doesn't much matter.
In fact, how music is perceived does matter -- your can't get the visceral stimulation of a booming subsonic bass with a headset, and the excitement of a live gig shared with thousands of other fans is lost in solitary listening -- but it matters less than with books.
So what? Digital music (delivered on iPods etc.) will continue to grow, and digital books (read on iBooks etc.) will continue to languish. Video will be a good test case of this theory; it's both visual and aural. I think video is closer to audio than to books; film-makers have told me that audio is an often-underrated part of their impact. They are already consumed in many forms, from cinema to portable DVD players, and this diversity will continue to increase. In some case, the medium carries more of the message than in others.