The best way to "protect" copyright these days is not DRM and other kinds of limiting measures.
If you want people to have incentive to buy the book from *you*, you will need to think of ways to offer a superior product.
As you cannot change the content of the book, you can think of adding value in the way of services to the book.
Perhaps extra info that is not included in the book.
That's the way it is usually done.
People go where they feel they can get the most value.