Send the id of the author along with the book. Have a separate AuthorProxy that manages the authorList. Have a BookProxy method that gets the author for a given book by retrieving the AuthorProxy and asking for the author with the id given in the book object.
Also, try not to use Commands to maintain the integrity of your Model. They are for Business logic, not Domain logic. I'd suggest a quick read of the best practices document to clear up most questions about where stuff goes