"The Reluctant Empress" by Brigitte Hamann

It's available in English and German, possibly French as well.

It's highly regarded as it delves into disproving many of the romantic myths surrounding Sisi and really delves into the complicated woman she was. The book is also sorted by themes (ie. her beauty routine, her relationship with her children, her traveling) versus chronologically and I feel like everytime I re-read it I have a newer and deeper understanding of who she was.