Robert Massey has two outstanding biographies on Russian rulers; the first being of Peter the Great, and the second being of Catherine the Great. The two are somewhat like companion pieces, though there is a gap of forty years between the reigns of the two. My personal favourite is his biography of Peter, which ranks as being one of the most fascinating and engaging biographies I've ever read (and won the Pulitzer the year it came out). Massay has also written a book on Nicholas II and his wife Alexandra, though I haven't read it so I can't comment on it. But Massey is highly knowledgable in the topic of Imperial Russian history, and also has a very accessible and readable writing style.