I only just saw this and the news must have totally skipped over me. I'm so sad to hear this, I must have first read Nicholas and Alexandra when I was 12 or 13 and it was always consistently important in my development on this subject.
It was such an important book for the way in changed both the popular and academic fields, bringing them together and sparking renewed interest in the subject of Russia's last imperial family and the world in which they lived. Writing subsequent works on Russian history, he continued to contribute to the field, even in the last ten years. I am particularly saddened that he died from complications with Alzheimers- I hope now he knows again what an important contribution he made.