“Alone : Britain, Churchill, and Dunkirk” by Michael Korda offers readers the history behind the popular film “Dunkirk”.  British troops , cut off by the Nazi invasion of France  retreated toward the coast, then were evacuated, famously, by a flotilla of both military and civilian boats.  Korda  brings a personal touch to the story: his own family had found shelter in Britain after fleeing from the continent a step or two ahead of the advancing Nazis. His is “a swiftly paced, illuminating account of these dramatic events.”