The Mermaids Singing

First book by Mcdermid that I've and I quite enjoyed it, not too difficult a read unlike a lot of other crime novelists whose first in a series is often a bit labored.
I did guess what was going on quite early on, mainly due to how the killers 'diary' entries were presented, but I never know whether that's the authors intention or not? Are they deliberately letting me feel a little smug?