Peter Pomerantsev is an authority on propaganda—and counter-propaganda. He is a Soviet-born British writer and teacher. His latest book is “How to Win an Information War:  The Propagandist Who Outwitted Hitler.” That propagandist was Sefton Delmer, a fascinating personality. World War II offers parallels to our own day, of course. Pomerantsev is a master of a slippery and critical subject.