Some interesting concepts, but I never really got over my irritation with how stupid the characters seemed to be in the first third of the book. Why do they have to lie and conceal things from each other for so long? It made the concept of historians travelling back in time to observe history less convincing, since they seemed so completely unprepared and out of their depth. The ending improved the book overall, and wrapped things up very neatly, but I feel it dragged on a few hundred pages too many to get to that point.