Add short titles to main books

bibliography.bib

@@ -1,6 +1,7 @@
 @book{burkehb,
  author = {William Hastings Burke},
  title = {Hermanns Bruder},
+ shorttitle = {Hermanns Bruder},
  subtitle = {Wer war Albert Göring?},
  location = {Berlin},
  publisher = {Aufbau Taschenbuch Verlag},
@@ -12,6 +13,7 @@
 @book{wylliewr,
  author = {James Wyllie},
  title = {The Warlord and the Renegade},
+ shorttitle = {The Warlord and the Renegade},
  subtitle = {The Story of Hermann and Albert Goering},
  location = {Cheltenham},
  publisher = {Sutton Publishing},