I read this close upon finishing Gregorio's first novel 'Critique of Criminal Reason' and found it as good.
'Days of Atonement' is set a few years later in time: it's October 1807, and since the battle of Jena (October 14th 1806) East Prussia is living under occupation by Napoleon's armies. Lotingen too, where Hanno Stiffeniis lives, has been occupied by French troops, and when an entire family is found murdered in a hut in the woods he must investigate.
As in 'Critique of Criminal Reason' this 2nd book takes place during the harsh Prussian winter, which adds to the bleak atmosphere (the crimes described are pretty gruesome too), but whereas 'Critique' was set in the old city of Königsberg with its tiny streets and alleys, giving that book a sense of oppression, Lotingen and the places where Stiffeniis must go in the search for this killer convey a sense of endless, desolate wastelands.
Neither are there a lot of 'shiny happy people' in the book, on the contrary. Gregorio, in his description of the characters and their lives, has created a virtual rogue's gallery as to almost virge on the unbelievable (though I've no doubt life was indeed pretty rough in those days and survival of the fittest was no idle term).
All in all, a very rewarding novel, and I hope this doesn't prove the last about Hanno Stiffenis.