There are two reward levels that contain limited supplies of books signed by all three of us. As of this writing the one remaining copy of Sleep signed by Eric has already been grabbed, and there are only a small number of signed Leonardos remaining. So act early!
If you do not get to subscribe in time to get your book(s) signed by Eric, then the only way to get one is to watch for possible stretch goals that open up that reward again, or to attend a concert or other appearance by Eric and have him sign your book then. What can we say--he's a popular guy!