"A strong and unusual story which works on several levels: an adventure in time travel, the quest for a missing person, and the search for a stronger 'self.' The book is one of thosse that leaves one with new images and unexpected thoughts long after one has finished reading." -- Eva Ibbotson, author of The Secret of Platform 13 and Which Witch?
Nice elements of this book: African American genius twins, substory about coming to grips with a terrible loss, The French Revolution, engaging main characters.
note: I think the book description misrepresents the book a bit by emphasizing "an enemy" - do recommend for 9-12 year olds.