Is there a recorded copyright assignment from them personally to the company? If not you may have to initiate litigation to stop them from asserting ownership in this copyright, Likewise, if your LLC Buy-Sell contained a non-compete clause, you may have to sue to enforce it.
If you did have a lawyer to form this corporation, register the copyright this company's based on, handle the Buy-Sell of the company stock and its assets, etc., contact that lawyer. If you didn't think proper legal work was important til now, you now know you need a business/IP litigator.
Avvo doesn't pay us for these responses, and I'm not your lawyer just because I answer this question or respond to any follow-up comments. If you want to hire me, please contact me. Otherwise, please don't expect a further response. We need an actual written agreement to form an attorney-client relationship. I'm only licensed in CA and you shouldn't rely on this answer, since each state has different laws, each situation is fact specific, and it's impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue.
Sounds like you will have to possibly file for declaratory relief. Consult an attorney.
Information provided in this response is intended to be informational or educational only. It in no way establishes an attorney-client relationship. Because every case is factually dependent, it is not possible to accurately answer each question posed. If you have sincere legal concerns, it is highly recommended that you seek legal counsel in your area. A response is not intended to create, nor does it create, a continuing duty to respond.
I don't know which version of the transfer rules North Carolina's Circuit Court follows; depending on that, you may or may not have received the copyright by assignment and will probably have to go to court to resolve the issue. Hire a local IP attorney to help you figure this out; it's a very complicated doctrine with at least three major regional versions.
Basically, your author has a valid copyright in the work unless he or she signed a work-made-for-hire contract before beginning work on the book. Given the situation, though, I'm guessing you two went the "fair sailing means no lawyers" route from the beginning and are now dealing with the fallout of not properly establishing your legal relationship.
So, if the author owns the copyright, then the question becomes "what language was used in the sale documents," because that's going to determine (along with your Circuit's take on 201 and 202) how this plays out. Some courts hold that a sale of all business assets to another party will include all of the associated intellectual property where the economic reality is that the business would be useless without them - this is probably best for you. Some others just say that it's possible for such a sale to transfer copyrights but do a case-by-case analysis to determine whether to transfer or not - this one is still okay for you. Some courts, though, say that copyright transfers have to follow the black-letter language of the Copyright Act, which says that no transfer of copyright is valid unless in a signed and executed writing specifically stating that copyright will transfer - the best you can get absent such a writing, in those courts, is a license. That would probably be bad for you because I'm guess you didn't go to a lawyer when your partner sold out, either, and so no one told you how to write the purchase agreement so that you could be sure to get the copyrights.
So, the short answer? You really, really need to speak to a local IP lawyer to figure out what's going on here. You should have done so when you started the partnership and when your partner sold out, too, because the best a lawyer can do for you now is to perform triage on a horrible situation and it's likely to cost you more money and not get you the results you would have gotten if you'd just had it taken care of in the first place.
No information you obtain from this answer is legal advice, nor is it intended to be. You should consult an attorney for individualized advice regarding your situation. No attorney-client relationship is formed by my responding to your question.