Last Enemy by H. Beam Piper (1904 - 1964)

An undercover Paratimer has disappeared on assignment while in an alternate time line, and it’s up to Verkan Vall of the Paratime Police to save her. To do so, he must infiltrate a universe in which assassination is an honorable profession, and reincarnation a scientific fact. Will Verkan Vall survive in a world of killers and the undead? (Summary by Mark Nelson)