A: If you co-authored the books, then you could be a joint author and copyright owner with the deceased writer. Giving your co-author their attribution would be appropriate and giving their estate their portion of any proceeds due to the sale of the books is also appropriate. More facts may be needed to answer this question, because the first step is to determine who owns the copyright of the written works, then based on that answer a copyright attorney can help you take next steps.
A: It depends on the specific language of the contract. There also may be other ways of terminating this contract if that is what you want to do, but again, it depends on the provisions found in the contract. It would be helpful to have an attorney review the contract to understand all your options and counsel you on taking the most appropriate course of action for your situation.