iBooks no longer obvious in iTunes

Not sure about how it looks on Windows, but on a Mac the Audiobooks icon (if you have it enabled) shows you the audiobooks books that you have in your library, and the iTunes Store link on that paper of your library takes you to the books and audiobooks section of the store - those icons, which allow you to access the different sections of your library (and store), are the only icons that you can customise.

But as Macs on Mavericks or Yosemite have their own iBooks app, that is presumably where Apple expect you access the iBookstore, not from iTunes.

With Mavericks (and now Yosemite and iTunes 12) iTunes is effectively just a tool for syncing books to a device, the management of them (including putting them into collections, reading them) is done via the iBooks app - so to a degree having that as the main place for accessing the iBookstore makes sense. What it's like Windows I don't know, I haven't managed to find any info..

Once you are in the store in iTunes you can access the different content types via the bottom left of the store's pages

In terms of Apple, these are user-to-user forums, if you want to leave feedback for them : http://www.apple.com/feedback/itunesapp.html

For ages people have complained about iTunes being too bloated, by separating books out into a separate iBooks application on the Mac. I think this will address this concern.