The Collector by Daniel Silva

 Even though Gabriel Allon has retired from the spy game and is focusing on art restoration, he can't help but get pulled back in. He's asked to find a missing Vermeer painting which gets him involved in stopping  a Russian plot to use a false-flag operation in Ukraine. Gabriel's partner in crime this time around is a Danish thief/cybersecurity expert called Ingrid. It's quite the page-turner.