I don't think you are talking about the same book-tho' that book does sound interesting! (maybe "Queen Victoria's Relations" by Meriel Buchanan that is discussed on the Buchanan thread). I believe this book is composed of letters between Alexander I, Nicholas I, Constantine and their sister Anna between 1817-1855.
I went ahead and plunged! I will let you know how it is when it comes. With me, you usually can't go wrong with diaries or letters. I can read and re-read books like thse.