Not a bad thriller.
If you've read books like Straw Men by Michael Marshall then I suppose there are similarities.
The difference I suppose is that there are really two timelines in this book. The main character, Sarah, was abducted and kept captive thirteen years ago. Now, her captor is up for parole and in order to make sure he doesn't get released she needs to contact her fellow victims. So we have a kind of investigative story set in the present and an unfolding of what actually happened years before.
I enjoyed it although there were some points where I wondered about plausibility.
Would an FBI agent, even a very sympathetic one, give over information so easily? Would they allow them to leave after the release of the women from the van?
It was interesting that Jack is always in the background but he never really gets an active role. Definitely not in the present and even in the past the action is all through the women's eyes. I don't think there was any dialogue from him. The letters I suppose.