Isn't authors selling the rights to their work basically how the entire book industry works?

That's the copying right, not the moral right. The moral right is the right to be identified as the creator and the right to maintain the integrity of the work in subsequent copying.

So, they can't sell that, as the GP is proposing, or they generally just don't under the current economic system? Because I'm pretty sure Dean Koontz does a variation of this, where he outlines the story but someone else actually writes the book, and it's published under his name.

That's a work-for-hire, in which the authorship of the work (and thus the moral rights) is agreed by all participants before the work starts to be FooCorporation.

Classic work-for-hire includes ghostwritten novels and autobiographies, corporate reports, web pages, and most likely every line of code you've ever written while employed by a corporation.

There are jurisdictions where moral rights cannot be sold, and others where they can, but work-for-hire nearly always sidesteps the issue.

It depends on the jurisdiction. In some countries like the UK they can be indeed waived, but in others like France moral rights are inalienable -- any contract wherein you give up these rights is null and void by law.

Selling the rights to publish your book is different from giving up all the rights you have over your book. The latter is something that (IANAL...) you can't do in many jurisdictions.