iTune dominates the music market because individual sales are cheap. Everyone can spare a dollar or two on a whim to try a song. (And since the removal of DRM, those songs play on any devices.) Trying the same locked-in software for ebooks at $13-15 each, when the books aren't readable on a number of devices people already own, is going to be worth watching. With popcorn.
People who had non-Apple mp3 players before, could burn the songs to CD and then rip the CD back to digital. It was a nuisance workaround, but it was available. Computers don't come with automatic burn-then-rip-this-ebook hardware & software. And the songs they had often weren't available elsewhere.
With ebooks, Apple is competing with Amazon, CoolERbooks, Barnes & Noble, BooksonBoard, and maybe Fictionwise; people will be checking which store has the cheapest ebooks. (And Apple's contract says they do. Which will limit what books they can get; they won't be able to offer Fictionwise's range.) They'll try to capture the mainstream-pop market, but they'll have to ignore the indie publishers--and any content they can't host, becomes lost customers. Especially since sites like Fictionwise, Smashwords & Webscription have *simple* interfaces--click here to pay; click here to download; no special software required.
Popcorn. I need popcorn.