For some reason, publishers in the UK and US have chosen different titles for these books. Mercy (which I received as an ARC some years ago) is the first book in the Department Q series; when published in the US, the name was changed to The Keeper of Lost Causes.
The Purity of Vengeance was published as Guilt in the UK, but I also found it on Goodreads with its original Danish title--Journal 64. Titles are important, of course, but all you really need to know is that if it is a Department Q novel by Jussi Adler-Olsen, it is going to be good.
One other minor complaint is that Amazon doesn't always include the name of the translator. It is a hit or miss kind of thing, and any work of translation should include the name of the translator. I don't believe the humor in Adler-Olsen's books could be as successful without a skillful, an artful translation, and it is the humor in the books that truly sets them apart. The books are all great crime books with loads of tension, but the humanity of the characters and the humorous bits of comic relief make the tension bearable and lift the books above the typical Nordic Noir/Scandinavian Crime novel.
Back to the book. The Purity of Vengeance (or Guilt, or Journal 64, whichever name you choose) was inspired by actual events on the island of Sprogo, where young women deemed unfit to reproduce were given abortions or sterilized without their consent and often without their knowledge. From 1923-1961, the "pathologically promiscuous" were sent to Sprogo in an attempt to keep them from breeding more degenerates. (! Hard to believe, isn't it?) The picture below is from the 1950's.
When Department Q (the cold case department in Copenhagen) gets involved in a number of old missing persons cases that occurred on the same day, similarities begin to appear. Piecing together the connections of five missing individuals who, on the surface, have no connections, keeps the members of the team relying on their feelings as well as the few facts they gather, and they dig deeper in their efforts to discover what happened. There are numerous sub-plots that the author skillfully manages and that keep the reader darting from situation to situation, from present to past and back again. And things aren't always as they seem.
The humor is inserted in the relationships among the Department Q members (Carl Morck, Rose, and Assad), friends, and family. The balance between evil and comic relief is so adept, so perfectly timed and deftly handled, that what could easily be overwhelmingly discouraging about human behavior is remedied by the humor the author uses to break the tension.
As expected, I was involved from beginning to end, appreciating all aspects of the novel: anger, apprehension, horror of what happened to the women considered unworthy of reproducing--and the eccentric, kind, mysterious, and amusing behavior and remarks of the cold case team.
The Department Q novels are the very best of Scandinavian crime fiction. Highly recommended. (I ordered The Keeper of Lost Causes, the first in the series, for each of my daughters, and I hope they will continue the series.)
Crime/Police Procedural. 2013. 512 pages.