This is the second in the Chronicles of St Mary's series. The first book set up the concept of St Mary's - a centre for historical exploration. The members use time travel to chronicle what happened in the past. Obviously there are bad guys out for their own gain.
This book has a few threads but builds up to an expedition to not just find out what happened then, but to head off the bad guys. It's a fair chunk of a book and at about 3/4 of the way through I was thinking there might have been one too many "side trips", but everything did lead to where things were going.
In the first book Max, the protagonist was a bit unsure of things but this time round, she's hardened up and at time shocked me with her behaviour.
In the main, this is a fairly light read, but gets serious at times. I really enjoyed it and like this author and the world she's created. I was thinking of this when I wasn't reading it.