This is far from the best novel in the Commissario Ricciardi series.
I think that the novelty of Ricciardi’s special curse has worn off for the author, and maybe for his readers as well. More than in any of the previous novels, “The Deed” is represented as madness, rather than as a supernatural curse (or gift). And unlike all the previous novels, “The Deed” plays no role in the solution of the murder mystery. In part that is because the murder itself took place months before Ricciardi begins his investigation. But that fact, of course, is a choice by the author, and I think represents his withdrawl from the core concept.
Regular readers of the series will recall that Ricciardi’s tata Rosa died in the previous novel, leaving Ricciardi even more alone than before. He is experiencing an unbearable depth of grief, and is withdrawing even further into his own loneliness. In part because of this he makes a definitive break with Livia, the beautiful, wealthy, and connected woman who has been pursuing Ricciardi for months. His break from her leads to consequences that he could not possibly have foreseen. And he is still devastated by his discovery in the previous novel that Enrica, the neighbor woman that he has (somehow) fallen in love with, from afar, has a new suitor - one that she seems to have accepted.
I see this novel as a transition to what, I predict, will be the final novel in the series. Ricciardi in Glass Souls has reached a crisis, and the resolution of that crisis will determine the course of the rest of his life. We are given hints as to how the crisis will resolve, but nothing definitive.
I look forward to the next novel in the series.
Update (August 23, 2017): I see from https://www.goodreads.com/series/8660… that there are already 3 more in the series, not yet translated to English, so plenty to look forward to.