I enjoyed this mostly for the characters; Archie because he's struggling with pain and loneliness and isolation, Susan because she's wanting and needing things she doesn't yet have. The secondary characters, even those who only appeared briefly, also had life. I will likely read the rest of this series (three more so far) to see if there's more of them. I think the serial killer, Gretchen, is also in the later books - she's a sort of Hannibal Lecter character, with extra doses of manipulation.
If you don't like icky stuff, I wouldn't read this. The present-day story is interspersed with Archie's torture at Gretchen's hands; it's not hugely graphic, but what was done to him is described, and it's pretty inventive in a gross way. I'm told the series gets a bit more graphic as it goes on. This one was less vivid than the Tess Gerritsen I read a few months ago, if that helps.
This was terrific reading for while I was on the elliptical trainer. Then I went home last night, afterwards, and finished it.