There are a lot on the ruling families. I like "The Last Empress" by Gorden Brooke Shepard on Empress Zita. He also wrote books on Franz Ferdinand (Archduke of Saravivo), Emperor and Franz Josef. One should start from his books. A recent addition is John Van Der Kiste book on Emperor Franz Josef, a more entry level book that cover the basics. As for Sisi, there are at least six that merits mention. But the one not to miss is the Corti one, filled with first hand information.