Emily Williams, who blogs over at Digital Book World, made an interesting observation in a recent post: she says "Stripping away the paper form has revealed the true nature of books: they are complicated bundles of copyrights. It is this fact even more than the centuries-old maturity of the print book market or the robust codex form (look ma, no plugs! no compatibility issues!) that has slowed the transition from print to ebooks."
She goes on to talk about how the images associated with a book, like the cover art, must be licensed both for digital use and international distribution. And for nonfiction books that have photos and illustrations it's a headache for the publishers who have to deal with not just author copyrights but the rights to all the images.