Options:
- Set up an Amazon Household with the other person. They can then share books from their account to yours so that you can continue to access them. You will need to agree to share payment methods, and only two accounts can be linked. See https://amazon.com/help/familylibrary
- Take over the existing account. This would only work if the other person does not have digital content and order history and such that they want to keep.
- Start over with your new account, giving up access to your previous books.