I do love Department Q. This one is one of the better ones in the series, and it was enjoyable. Twisty turny investigation path, good characters, all the good stuff.
I've got to say that the structure of these novels is a lot like the TV show Criminal Minds. You spend a lot of time with the team working the case, but you also have interludes with the sociopathic/psychopathic villain. I suppose these interludes are to increase the urgency for the case -- you know that this cold case is not over and done with. There are related very bad things going on, and if they work quickly enough, they just might save a life. The down side is that we know who the villain is, and they're not fun to spend time with. I find the villains in both this series and the TV show a little, umm, exaggerated. And I don't like the interludes. I'd prefer the books without them.
BUT, nobody asked me how to write these things. And the finished book, with my favorite character Assad featured prominently, is interesting and fun. And I appreciated a bit of a twist at the end -- it didn't turn out quite the way I had figured. Or how other characters had figured. So that was why I say it's a little better than some of the other books in the series.
I got a free copy from First to Read.