The code of love - Andro Linklater

In 1939 Pamela Kirrage, headstrong and beautiful, met Donald Hill, a handsome RAF pilot. But Britain's war with Germany meant their plans for marriage had to be postponed. Hill was posted to Hong Kong, and sensing that he was caught up in the sweep of great events, he began a diary. However, officers serving abroad were forbidden to keep such records, so Hill devised a secret code that transformed his words into numbers. The Donald Hill who returned from the war, after four years in a prisoner of war camp, was a dramatically changed man. Tortured by savage flashbacks, he was confined to hospital. Pamela had always known that the numbers in the school book were a code, and that somewhere within the lines of figures she would discover the key to understanding Donald's mental collapse. Eventually, after Donald's death, she found a mathematician who was able decode the diary and reveal its account of Donald's suffering in the camps.