You buy a physical book (also called "a copy of the book"). You don't make the copy, you buy it from a legitimate seller.

Then you can modify that physical book, and sell it again.

If you want to sell a second one of the work you made, you need to buy another physical book (another "copy of the Hobbit"), modify that physical object in the same way, and sell it again.

You would own the copyright to your changes, so no one else could modify "the Hobbit" the same way, and no one else could publish instructions on how to do the modifications yourself.