All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
There is nothing wrong with displaying your book on the mentioned website and availing yourself of the money earned by selling it or through your rights as the author.
What you mentioned, that the website is owned by a non-Muslim or that the source of the purchaser’s money is unknown or the like, affects neither the permissibility of that transaction, nor that of availing yourself of the earnings. It is permissible to engage in financial transactions with non-Muslims because the Prophet and his Companions did so. Moreover, it is not incumbent on you to investigate and search for the source of a person’s money; the basic principle is that it is free from what is unlawful.
Allah Knows best.