"This week marked the passing of Archduke Otto von Hapsburg, the eldest son of Blessed Karl of Austria—he was beatified by Pope John Paul II in 2004,—head of the House of Hapsburg and last crowned prince of the venerable Hapsburg family. He passed away in his sleep on July 4th at the age of 98."—from Unam Sanctam Catholicam July 6, 2011
The only biography in English that I saw at Amazon.com was Gordon Brook-Shepherd's Uncrowned Emperor—the Life and Times of Otto von Hapsburg which was published posthumously in 2003 and apparently not very academic according to some reviewers.
I knew a little about the Hapsburgs in various parts of Europe because I suffered through a survey course in medieval and modern European history while in pre-medical studies, but that name and my interest both petered out about WWI. In any event, though his father Karl had the misfortune of dying young and not really debilitated from "the friend of the aged," according to Sir William Osler, the Archduke lived long enough to just die in his sleep with little or no fuss.