After reading this book, I looked up what became of Princess Stephanie (Sisi's son's wife) and her daughter. From what I was able to glean, their lives were pretty terrible, being disinherited by father Leopold II, caught in the middle of two world wars, mother/daughter estranged, etc. It would probably make a more interesting book than "Sisi."
Also interesting to me because my grandpa was drafted into Franz Joseph's army back in the mid-1890's from Croatia. He had never been anywhere, but was assigned as a valet to an officer, traveling on horseback from castle to castle, delivering dispatches. He loved it, and tried to re-up when his two years' service ended, but wasn't allowed to; so, with all his experience from waiting around in castle kitchens, he took off for California and opened a restaurant.