The World of Yesterday by Stefan Zweig — Zweig's memoir of the Austro-Hungarian world before its destruction is the essential book for understanding why the Salzkammergut mattered so much to the Viennese bourgeoisie. He describes the summer migrations of artists and intellectuals to the lake district with the melancholy of someone who knows it is all gone.
The Hare with Amber Eyes by Edmund de Waal — The story of the Ephrussi family's netsuke collection and its survival through the Holocaust traces the same Viennese upper-class world that summered in the Salzkammergut. The sections on the Austrian branch of the family are set partly in the world of the lakes.
The Rings of Saturn by W.G. Sebald — Not specifically about the Salzkammergut, but Sebald's method of reading landscape through layers of historical catastrophe is the right one for this region. His way of finding grief in apparently pleasant countryside applies here with particular force.