Books can also be made with wood as their covers.  And diptychs and triptychs, which come from the Greek meaning two- (dip) or three- (trip) fold (tych) also fall into this category.  Historically they refer to works of art that are divided into panels and hinged together and usually featured religious themes during medieval times.  Artists now use this form across a wide array of mediums, including painting, photography, and sculptureAccordion Books