I wasn't really so into this one ... do like her work, and the mystery element was quite good (well crafted for running it on two timelines). It was billed as deleving into the backstory of the main characters, and well that might be the problem. I don't actually really care for them; Sara is a bit of a Mary-Sue if you look too closely. Also that it lost a lot of the pace for there being big chunks of 'this one time down the mine' or keeping track of heaps of minor characters (because the might be important later).
Bit of a low note compared to Slaughter's usual par, but still I've had worse.