Museum board is a special type of cardboard. Specifically, it is a white, acid free cardboard with the interesting property that it flexes fairly easily in one direction but it is fairly stiff in the other. This is because of the grain of the paper fibers. Denny Jones recommends two ply museum board for long stitch binding. Good art-supply stores carry this. One sheet will make about eight book covers for books printed on standard sized typing paper.