Leonidia,
The concept you are trying to wrap your head around is called privity. It is part of the traditional English Common Law, which in some ways is a rough proxy for natural rights or the natural law, particularly where it concerns property rights. One’s private property can only be confisgated in two situations:
(1) By consent (directly or indirectly, such as pursuant to a voluntary submission to an authority)
(2) By violating the natural, categorical or customary property rights of others, assuming the confisgation is proportional.*
In the situation you described, the contract is only between A and B. B promises not to copy or distribute the book. A and B promise that if B does distribute the book, C will pay. But C doesn’t sign the contract. Therefore, C does not fall under #1. Second, C cannot fall under #2 because, as discussed before, there is no natural right to an idea or expression–it does not inhere in either basic goods, the intuitive structure of life and free choice, or our axiomatic self-ownership.**
Instead, A’s remedy would be all damages caused by B as a result of the distribution, but only if it is in violation of an express term of the agreement. At some point, B won’t want to be liable and won’t sign such an agreement, so A & B will negotiate terms among thousands of other A’s and B’s, until the market adopts a general risk balance between writers and consumers (where both A & B make sacrifices to make it worth both parties’ while to write books and enjoy them). Furthermore, to the extent people use DRO’s, such organizations will almost certainly include the “pledging” of a significant portion of one’s assets – i.e. the DRO may very well be your bank. In which case, buyers and sellers are more willing to deal with one another, as a neutral 3rd party already holds the funds you would want to confisgate in the event of a breach of agreement.
*A proportional right to confisgate property must be tempered by the amount of damage or chaos one might cause in attempting to enforce such a right. The need to invade someone’s house to steal a television because someone owes you a debt would probably render a response disproportional, at least to the extent it risked a basic objective good, like someone’s life. However, as discussed above, the market is able to avoid such qualitative quagmires with clever solutions applied to the front-end of a transaction; culture, particularly Christian culture, plays a part in providing for more humane debt collection. Even modern collectors will engage in safer tactics (like repossessing a car in the middle of the night) to remedy a debt.
**Of course, the following only concerns C’s obligations as a matter of justice. If C knew of the contract between A & B, he may feel obliged as a matter of charity to respect A’s desire for payment and shortsighted distribution of his work without other protections. In fact, in the real world, many people’s decisions are based on charity and goodwill rather than mere justice, despite the common confusion of the two concepts.