Carolina De Robertis’ fifth novel, “The President and the Frog,” revolves around a backward look at the life of the aged, unnamed protagonist, a former revolutionary in a South American nation who underwent long years of torture and imprisonment before emerging to ascend to high office, becoming both beloved by his countrymen and revered around the globe for his humility, compassion and lifelong thirst for justice. The key to his survival — if not, perhaps, his hold on sanity — turns out to have been his recurrent and frequently fractious conversations with a thoroughly obnoxious frog that comes and goes at its own whim.