I managed to find your options somewhere on the Internet: ebooks, hard cover books, mass market books, and paperback books.
I believe the most expensive form of books among these options are hard cover books. First of all, they take longer periods of time to be made; secondly, they need firmer materials because their first page is not easily bent as that in mass market books (which is just a smaller and cheaper version of paperbacks) or ebooks, which are electronic, so it doesn't take a lot of money to make them.
