The Truth About Rasputin
This book was originally written in Russian and was translated very well, but as a result the writing style seems almost old-fashioned. If you can get past that and the confusion of large numbers of Russian names and relationships, this is good book.
This is the first book I've read that shows Rasputin as human, and with the aid of new information, gets to the heart of the situation at the end of the Russian Empire. Although the historical backdrop is huge and the times catastrophic, these are still just people with normal human motivations and feelings. The author indulges in a little bit of speculation and retrospective psychoanalysis to arrive at his conclusions, but there's really not much else to go on at this point, and most of his conclusions seem sound.
Parents, some of the themes are adult in nature, but they are handled very sensitively. A high schooler should be okay with most of the material. There are pictures of Rasputin's dead body, but they're not terribly gory. There is at least one political cartoon that is obscene.
I enjoyed the book and would recommend it for anyone who has been curious about how this illiterate peasant got so close to the throne of Russia, what his goal was, and why the royal family remained so fanatically loyal to him even in the face of certain tragedy.