The Pendulum — Edward Cline

It is late November 1929. Cyrus Skeen has concluded his case in Reprisals, and apprehended the elusive Gideon Knowles, the Nazi enforcer of a vendetta concocted against Skeen because in the previous month he exposed an American Nazi Bund near Stanford University. Knowles had escaped arrest, but he has returned and has threatened to kill Skeen. Skeen tracked him down to a San Francisco apartment hideout. Skeen foiled one assassination attempt by injuring the man, and killing a second assassin who had murdered the first assassin. But the Germans have not yet finished with Skeen. An official from the Berlin government's special delegation emerges. He is not a Nazi, but another who is determined that the Fatherland will be avenged for its humiliating defeat in WWI. He is assassinated, but the intrigue only begins.