Thanks for recommending this, OP. It's quite enjoyable. I would add, though, that it's really only on interest to complete royal watchers who know more or less who everyone is in the tangled and massive Windsor and Mountbatten family trees; otherwise you'd have a hell of a time knowing who people are and how they're related to one another.
One of the best things about the book is that Pope-Hennessy has absolutely no illusions about who these people are, and he thinks basically they're all a different species than other human beings because of their bizarre upbringings. but he's also a deeply kind man who warms up to them very easily, and so they all pretty much dote on him and want to hang out with him until the wee hours of the morning. What's also interesting is that since he's pulling the whole biography together he actually knows more about Queen Mary than anyone he interviews, so that even her own son (the Duke of Gloucester) begs him to tell him secrets from his childhood he never understood, for example, was Mary's first fiancé, the elder brother of her eventual husband George V who died of influenza before they were married, syphilitic (the answer: almost certainly) and gay or bisexual (the answer: likely).
Somewhat surprisingly given all the negative focus the Duke of Windsor has received in recent decades, Pope-Hennessy likes him the best of the royals he meets, and finds him the kindest and the smartest, and laments the waste of his life. He likes the Duchess of Windsor, but finds her a Southern belle who is pretty shallow but with a marked talent for dressing and for interior decoration.
He also surprisingly does not care for the current queen, who commissions him to write the biography of her grandmother. he thinks she's much like her grandmother (intensely intellectually curious, and with a great memory), but too tightly wound. Later, however, he hears that the queen finds him "cold," which surprises and disappoints him because the rest of the family seems to really like him.
Interesting facts in the book:
*Queen Mary's famous deep reserve in public belied the fact that personally she was quite warm to most people... except notably her own children. They pretty much regard their father, George V, as a terrible father because he bullied them and yelled at them, and they think her "a oral coward" for never taking their sides even when they knew she knew their father was wrong.
*Queen Mary was much disliked by George V's mother, Queen Alexandra, and by his favorite sister the unmarried princess Victoria, who was George V's favorite. Part of this seems to stem from the fact that she was only born "Her Serene Highness" rather than royal (as Alexandra and her daughter were). But people in the family are still puzzled by this because Alexandra was supposed to be such a nice person otherwise.
*Another of George V's sisters, Louise the Duchess of Fife, was so stupid as to be considered practically retarded by the rest of the family.
*Royals in Europe in the Fifties seem to live in two ways: either they indulge themselves in the finest food and luxury imaginable (as with the Windsors and Prince Axel of Denmark), or they deny themselves good food and even decent indoor heating so as to mortify themselves and save money even when don't need to do so (as with Mary and the current queen).
