How did Nathaniel uncover the secrets of his mother's life in the novel, Warlight?

As an adult, Nathaniel worked in government archive offices in London. Nathaniel was employed in altering and redacting various government files relating to secret espionage military actions undertaken by the British government both during and after World War Two. Nathaniel decided to secretly seek out files relating to his mother’s espionage work. He used these files to construct a general narrative of his mother’s secret life.