Hey, you want Apple's executives to have to take night jobs to pay their electric bills? iBook Author exists to create ebooks for the iPad, ebooks that are sold through the iBookstore. Eventually, they may make those ibook files readable on iPhones (sorta) and on Macs (we all hope). But for now, it's just the iPad. Even that page size you see is an iPad's screen size. That's why it can't be changed.
Proofing is probably the main reason why Apple added the Share menu item with the option to save or email a PDF. That PDF can be read by all sorts of apps. That's what you should send to your proofreader. I've not checked, but I'm assuming the layout on that PDF is identical to that on an iPad. So a proofreader should be able to reference a typo by referring to "page, 3, second full paragraph, third line."
Apple knows what it is doing. As an author, the release of this app is making me think much more seriously about getting an iPad--that and the fact that there's an iPad version of Scrivener under development.