This last in the trilogy was a bit of a curate's egg, really. The first two books are a fair balance of mystery and back-story, with perhaps a little more emphasis on the protagonist's life story and motivations than is usual in most 'detective' novels than usual. This worked well for me, not only because it was heartening to read about a man in a, traditionally, 'tough-guy' vocation having emotional issues and experiencing fear without the author resorting to the usual sort of clipped, masculinist cliches in order to convey this, but also because of the descriptions of life on Lewis. Where this third falters a little, perhaps, is in the every-other-chapter approach to the past and the present. Switching between timelines is not of itself a fault in a novel; some books are constructed entirely around this. However, the pacing of this one is off to the point that when the reader gets to the alternate time line of the next chapter, the effect is not; 'Ooh, more information to shine light on what is happening and why it's happening - goody!', but rather; 'Oh no, not another flashback....get on with it!' The balance between back story and mystery is consequently, rather overthrown.
There are other issues, however, and that is that even though Scotland may have the highest murder rate in Europe, the body count is rather high and the events rather too coincidental to the hero's own life. I don't want to offer examples as these would effectively be spoilers, but really, the various denouments towards the end are just too much of a stretch.
In fairness, I should say that I have enjoyed Peter May's Lewis books overall, and that even with their flaws, would give the trilogy overall a 4 star rating, and recommend them to friends looking for an absorbing read.