If this happened to me, I would register some of the artwork with the Copyright Office. The forms are very simple and available at www.copyright.gov. It costs $35 to file a copyright application. If it is infringed after you file the application, you can receive statutory damages and the infringer can be responsible for paying for your attorney. Save a copy of the application(s) you file.
I would also send the author a letter, saving a copy, informing the author that he is not authorized to use the work.
If, despite your letter, he does use your work, contact an IP attorney to resolve the matter by getting an injunction and/or damages.
Answered on Mar 14th, 2014 at 1:20 PM