Involuntary Witness is the first title in Gianrico Carofiglio's Guido Guerrieri series. It has dropped to $0.99 at Kindle and Kobo US.
Kindle US:
https://www.amazon.com/dp/B004GUSBOC
Kobo US (coupon/VIPable):
https://www.kobo.com/us/en/ebook/involuntary-witness-2