I'd probably have given this a 3.5 star if the option was there. I did enjoy the book in general, but there were just a few niggles for me.
Firstly, I thought it went on just a little bit too long. By the time we knew who the kidnapper was, I felt it took a while to wrap up, and it seemed to carry on when the emotional climax had passed some time previously. Aside that, my main quibble was that I thought some of the plot points hung on the ineptitude of the police force.
I'm sure in real life this happens all the time, and I do not for a minute doubt the author's research, but for me personally, there were a few incidences that I lost a little bit of my suspension of disbelief, as I had guessed a plot point before it came due to somethings the officers seemed to not have considered, but this were very few, and in the main did not put me off from what was an enjoyable read.
Hayder's prose is generally excellent, and her characters sufficiently complex to keep a reader's interest. I was more drawn to Flea, the female Underwater officer, than I was Caffrey, whose own character didn't feel too deep in this one, but then I expect we learn more about throughout Hayder's other Caffrey books.
The Walking Man was both an intriguing character and handy narrative tool and Hayder used him brilliantly to alter and manage the pace of the book, and give Caffrey time and space to reflect and ponder. Really good stuff.
If you like realistic and superbly written police fiction, I think you'll really like this one.