This is working.

I think in your first comment you merged the idea contract and property into a single idea - and were calling that thing property.

To put my position another way, while you can have a contract to exchange property, not all things exchanged through contract are property.

    A single buyer could violate the contract and
    then the whole world would have access.

OK. So the scenario you describe is party A has produced a book. Parties X, Y and Z sign a contract with A to get access to the book with. The contract restricts them - it says that may not redistribute. In this case, if X redistributes, then perhaps A could sue for damages.

   The only way third parties could be prevented from
   getting it is if the law recognized this as property
   being stolen.

I see. And in a sense you're right. However, it's important to remember that creating that legal structure impedes other forms of innovation. For example, lots of music derives from other music. There are remixes, that are often illegal. A lot Handel's work is arrangements of existing music he had lying around. Handel rearranged it in a way that has survived the test of time better than the originals.

    The only way third parties could be prevented from
    getting it is if the law recognized this as property
    being stolen.

There are other mechanisms. Consider the Steam games network. That's a mechanism of protecting ideas that doesn't leverage copyright law. Another one was the way the 911 report was distributed. They sold right-of-first-access to that in an auction (for a large amount), and later released it free.

A lot of IP is created in a way that is incidental: music for movie soundtracks, code to make businesses work better, books so that teachers can teach their students.

The existence of copyright empowers people who are connected to the legal system. Musicians don't make good money off their music, the parasitic recording industry does. Authors don't make good money from textbooks, the parasitic textbook industry does. Copyright interests are particularly well-connected to lobbyists. The incentives argument doesn't hold up. People wade through rivers of shit to have creative expression, often in contradiction to their commercial interest, and are often frustrated in that goal by copyright-backed cartels that want to keep channels locked up.

I don't know if I'd call the textbook publishers parasites. Books with small audiences are more expensive to print.

Isn't a parasite something that takes resources against the host's will while providing nothing in return? How is that true here?

As with music, authors that appeal to large numbers of people have the most leverage. When distribution is very costly per copy, the take home for the producer will be reduced. Distribution is a real activity that needs to be performed. There's nothing parasitic about it.