There are a number of books which have been published that provide additional material on the key references here.
Georg Brühl, Die Cassirers: Streiter für den Impressionismus, Edition Leipzig, 1991
Marcus und Jeanette, Siegfried und Henriette Cassirer'sche Familien-Stiftung, 1924-1925, 1930, and 1931.
Toni Cassirer, Zum 18 Oktober 1937, gestetner produced "Anekdotenbüchlein über die Familie Cassirer" on the occasion of Max Cassirer's 80th birthday ,87 pp.
Toni Cassirer, Mein Leben mit Ernst Cassirer, Hildesheim: Gerstenberg, 1981.
Harry Nutt, Bruno Cassirer, Stapp Verlag, Berlin, 1989.
Peter Paret, The Berlin Secession, Cambridge, US, 1980; expanded German edition, Berlin, 1981.
Helga Thieme & Volker Probst (eds), Berlin SW - Victoriastrasse 35, Güstrow, 2003.
Dennis Shirley The Politics of progressive education, Harvard University Press, 1992.
The book details the Odenwaldschule (founded by Paul Geheb) in nazi Germany and Switzerland.
David, R. Lipton, Ernst Cassirer. The dilemma of a liberal intellectual in Germany 1914-33. Univ. of Toronto Press, ISBN 0-8020-5408-0.
Rahel E. Feilchenfeldt & Markus Brandis, Paul Cassirer Verlag. Berlin 1898- 1933, Eine kommentierte Bibliographie, K. G. Saur Verlag, München 2002 ISBN 3598115784
Renate Möhrmann, Tilla Durieux und Paul Cassirer, Rowohlt, Berlin, 1997.
Barbara Falk, Caught in a Snare: Hitler's Refugee Academics 1933-1949, History Department, Melbourne University, Melbourne, 1998†
Norman Davis, Microcosm: Portrait of a Central European City, Jonathan Cape, London, 2002.
An excellent social history of the city of Breslau (now Wroclaw) in the context of the history of Central Europe, and in particular Silesia, which formed the context for the modern origins of the Cassirers, Cohens, and Falks.
Numerous books on Ernst Cassirer and his work exist, and these can be found with a simple Web search.
Professor Peter Paret has written definitive historical analysis covering, amongst much else, the lives and works of a number of famous Cassirers.
The fullest account of Paul Cassirer as galerist, publisher, and secretary - later president - of the Berlin Secession is in Professor Peter Paret's monograph The Berlin Secession, Harvard University Press, Cambridge, Mass. 1980. Expanded German editions: Die Berliner Secession, Siedler Verlag, Berlin, 1981, 1982, Ullstein Verlag, Berlin, 1982.
The most recent analysis of Paul Cassirer's role in the conflict over modernism is in the collection of Professor Peter Paret's essays, German Encounters with Modernism, 1840-1945, Cambridge University Press, Cambridge-New York, 2001.
The most recent account of the collaboration of Paul Cassirer and the sculptor and dramatist Ernst Barlach is in Professor Peter Paret's essay "Bemerkungen zu einem 'seltsamen Freundespaar'," in Berlin SW - Victoriastrasse 35, ed. Volker Probst & Helga Thieme, Ernst Barlach Stiftung, Güstrow, 2003; and in his monograph, An Artist against the Third Reich: Ernst Barlach 1933-1938, Cambridge University Press, Cambridge-New York, 2003.
Professor Paret notes that the most reliable und up-to-date bibliographical information on Ernst Cassirer, Fritz Cassirer, Paul Cassirer, and Lucie Ceconi (Paul's first wife) as writers is in the new standard German literary encyclopaedia Deutsches Literatur-Lexikon: Das 20. Jahrhundert, vol. 5, ed. Konrad Feilchenfeldt, K.G.Saur Verlag, Zurich-Munich, 2003. Bruno Cassirer is not listed because he did not publish anything, Richard Cassirer is not listed because his publications were exclusively scientific, not literary.
Barbara Falk, No Other Home: An Anglo-Jewish Story 1833-1987, Penguin, Melbourne, 1988.†
Barbara Falk, "The Unpayable Debt", in Patricia Grimshaw and Lynne Strahan (eds), The Half-Open Door, Hale & Iremonger, Sydney, 1982.†
Henry A. Landsberger and Christoph E. Schweitzer (eds), They Fled Hitlers Germany and Found Refuge in North Carolina, Southern Research Report #8, Academic Affairs Library, Center for the Study of the American South, IRSS Faculty Working Group in Southern Studies, Spring 1996, pp. 69-74.
Philip Cohen, Henry Cohen (1790-1867), including an annotated bibliography (starting at Page 117).