Slate and shell are the traditional material for go stones.
Slate and Shell ( http://www.slateandshell.com/) is a publisher specializing in Go books. It was founded in late 2000 by three very active amateur go players Bill Cobb, Gordon Fraser and Chuck Robbins. You may order books online from their site: see
http://www.slateandshell.com/library.asp?cat=book.
Slate and Shell books (including books distributed rather than published):