As someone artfully conditioned by Eric Ambler and Philip Kerr and Alan Furst, I happen to be well-disposed toward novels dealing with the secret policeman’s Europe of the 1930s. But they have to get it right. It is not simply a matter of assembling a bunch of old Baedekers and street maps and railway schedules, or of cribbing nonfiction and academic research dealing with the period: art is required. An alternative reality that feels real must be brought into being. Craig Nova is one of those lucky writers gifted with the artistry necessary to pull this off. In “The Informer,” he gets it right.