Ask a Business Lawyer. Get Business Law Questions Answered ASAP.
I had to switch as I could not respond in chat.
There is no restriction on what you can negotiate with authors you publish. This is purely a question of negotiation. You can retain an exclusive market for some areas and a percentage of other rights the author retains. I suggest you have such a revision drafted by an attorney to make sure it does what you want it to do.
If this answer is responsive to your question, please accept it. That is how we are compensated. I would also be appreciated if you provided feed back on your view of the answer. Finally, if the answer was especially helpful you can provide a bonus. If I can be of further assistance or you have other questions in the future you can ask for me and reach me at this site.
This communication is not intended as legal advice. A local attorney should always be consulted for legal advice. No client/attorney relationship is intended or created by this communication.