I'm an Editor in book publishing. All opinions are my own.
I've recently read a couple disappointing novels, including this German bestseller by Nele Neuhaus, a perfect example of what I call the Overstuffed Mystery.
Now, I still haven't been able to figure out if this is the first in the series, or just the first printed in the US. If it's not the first in the series, part of the problem may be that some of the backstory in this novel occurred in previous installments, so for series readers that backstory wouldn't have seemed so intrusive. But even taking that into account, this story was filled with too many characters who had too many conflicts, and the mystery was overcomplicated in a way that ultimately felt implausible.
So my tip for mystery writers: the punch line of an elegant and compelling mystery does not need to be complex . . . the story just needs to seem complicated until the truth is revealed.