 Complete the following phrase:  In 1969 Kiefer began to design books. Early examples are typically worked-over photographs; his more recent books consist of sheets of lead layered with paint, minerals, or dried plant matter. For example, he assembled numerous lead books on steel shelves in libraries, as symbols of the stored, discarded knowledge of history. The book Rhine 1981 comprises a sequence of 25 woodcuts that suggest a journey along the Rhine River; the river is central to Germanys geographical and historical development, acquiring an almost mythic significance in works such as Wagners Ring of the Nibelungs. Scenes of the unspoiled river are interrupted by dark, swirling pages that represent the sinking of the battleship Bismarck in 1941, during an Atlantic sortie codenamed 