Günter Grass: The Tin Drum — the great novel of the eastern German territories, not set in Mazuria specifically but in Danzig (Gdańsk) and the Kashubian countryside. The Masurian world — Protestant farmers, Prussian order, violent displacement — is adjacent to this novel's world and illuminated by it.
Marion Dönhoff: Before the Storm: Memories of My Youth in Old Prussia — the Countess Dönhoff grew up on an estate in East Prussia (now Poland) and escaped on horseback in January 1945 as the Soviet Army advanced. Her memoir is a description of a world that vanished completely and the particular Masurian landscape of forests and lakes. Essential reading for understanding the postwar transformation of the region.
Wiesław Myśliwski: Stone Upon Stone — not set in Mazuria but a classic of Polish village fiction that captures the postwar experience of resettlement and rebuilding. The displaced families who repopulated former East Prussia came from exactly this world.