At the Existentialist Café: freedom, being and apricot cocktailsSARAH BAKEWELL(CHATTO & WINDUS, 448 PP, £16.99)Tablet bookshop price £15.30 • Tel 01420 592974 “Hardly a day goes by,” Gabriel Marcel noted in 1946, “without my being asked what is existentialism?” Nor was it only Marcel’s fellow thinkers who wanted the lowdown on the latest ideas. “Usually it is a society lady who asks,” he went on, “but tomorrow it may be my charwoman or the ticket-collector.” Ah, those heady post-war days, when abstract arguments were conversational currency for the world and his wife. When a philosophical tête-à-tête between caffeine-stoked Parisians becomes the subplot of an Audrey Hepburn and Fred Astai