By and large, people who make music are reasonably good at figuring out how to make decent music. They practise, they play for friends, they perform in small venues, they attract a following.
On the other hand, people who decide to write a book, often have no fucking clue how to write. Maybe they have a good idea, or an interesting story, or a unique perspective on certain events. But write a coherent well-structured book? Ha!
And the problem is, people think they can write. Let them all self-publish and many aren't going to believe they need professional help. Many musicians at least know they can benefit from a professional sound engineer, but how many first-time writers hire their own freelance editor?
Take book publishers out of the picture, and most auto-biographies would be unreadable. Large amounts of non-fiction would be unreadable, as would a surprising number of novels. Books that requires illustrations, would be filled with really crappy illustrations. Or none.
You think text books are hard to follow now? Leave academics to their own devices and see what you get.
Book publishers bring a lot more to the party than their music industry equivalents. They rewrite and restructure, fact-check, illustrate, do graphic design, obtain clearance for the use of quotations and excerpts, and translate to foreign languages.
I spent ten years writing custom software for book publishers, and I know that their's is one of the most complex and challenging businesses going. I'm not saying I agree with Macmillan's e-book pricing. But comparing them to the RIAA is ridiculous.