Not taking in consideration the ghost writer activity, how much is your book worth to you if somebody will offer to buy it?
Not just buy a book. Buy THE BOOK. Buy every right you have on it on perpetuity, including the right to remove your name as an author. That person will offer you an amount of money so you give up your literary baby, your work of art, your book completely.
How much will you ask for it? Is it a book as an entity priceless?
How much is your book worth to you?