There is an anthology edited by Charles Ingrao and Franz Szabo, The Germans and the East that covers German colonization from the Middle Ages to the modern era. If you speak German, Lothar Dralle's Die Deutschen in Ostmittel- und Osteuropa: Ein Jahrtausend europaischer Geschichte is another survey history. And although it covers only the modern era, Vejas Gabriel Liulevicius's The German Myth of the East: 1800 to the Present is an excellent study of the cultural importance of "the East" to Germans in the modern period.