To remove a co-author book, you go to the same page where you added them as a co-author:

...making sure to replace YOUR_BOOK with your book's unique web address.

In the list of authors, you will see a "Remove" link next to your co-authors:

Click the "Remove" link and you will see a warning like this:

Click "OK".

This will remove the co-author from the book.

(If you are looking for instructions on how to add a co-author, please read this article.)