That bibliography covers just the english titles of the books. As for the other languages, think about seeing the others if you are able to read them. Some works are only found in library or at the antiquarian booksellers, included internet. In red are marked the major accessible books.

BALFOUR Michael, The Kaiser and his Times, London: Cresset Press, 1964, 530 p.
Bogle : A heart of Europe : The Lives of Emperor Charles and Empress Zita of Austria-Hungary
BEECHE Arturo & MC.INTOSH David, Empress Zita of Austria, Queen of Hungary (1892–1989), (In memoriam collection), 2005, 66 p.

BOGLE James & Joanna, A Heart for Europe: The Lives of Emperor Charles and Empress Zita of Austria-Hungary, Fowler Wright, 1990, 176 p.

BROOK-SHEPHERD Gordon, The Last Habsburg, London: Weidenfeld and Nicholson, 1968; New York: Weybright and Talley, 1968, 358 p.Brook-Sheperd : The Last Habsburg

BROOK-SHEPHERD Gordon, The Last Empress: Life and Times of Zita of Austria-Hungary, 1892-1989, London: Harper Collins Publishers, 1991, 320 p.

CARROLL Warren.H, 1917 Red Banners, White Mantle, Christendom Press, 1981, 138 p.

HAMMERSCHMID, Leo J, Zita, the last empress of Austria : the story of Her Imperial and Royal Majesty Zita of Habsburg-Lorraine and her sojourn in Québec City, 1940-1950, Montreal: Meridian Press, 1989.

HARDING Bertita, Imperial Twilight- The Story of Karl and Zita of Hungary, Indianapolis-New York: Bobbs-Merrill Company Publishers, 1939, 388 p. (New edition: Brewster Press 2007).

POLZER-HODITZ Arthur Graf, The Emperor Charles(translated from German by D.F. Tait and F.S. Flint), Boston: Houghton Mifflin, 1930, 470 p.

VIVIAN Herbert, The Life of Emperor Charles of Austria, London: Grayson and Grayson, 1932, 309 p. (with the papers of Colonel E.L. Strutt)

Zeßner-Spitzenberg, Dr. Hans Karl von, ,The Emperor Charles I of Austria : A great Christian monarch ; A short history of his life and death,London : K. Rasinger,1963, 40 p. (Compiled and transl. by Katharina Rasinger)

Idem, The last Habsburg Emperor,London: K. Rasinger, 1965, 8 p.

Idem, Death of an Emperor,London: K. Rasinger, 1963, 40 p.

Thanks to Mr Julien Aumont for having worked on the layout of this bibliography.