The Children of the Company by Kage Baker

Another interesting Company novel.

This one raises some issues of causality that really made the series stand out. The rules of the world are set up for this, which is that time travel is possible but only backwards. So the Company's operatives know the history of the world up to 2350 or so, when they were sent back. The Company, to preserve causality, has set in stone the rule that nothing can be done that contradicts the historical record. In this novel, we see the operatives begin making things happen (often terrible things, e.g. engineered viruses) just to fit the observed facts. This provokes some thought about historical determinism (or not) which I found pretty good.