Precipice by Robert Harris

Summer 1914. A world on the brink of catastrophe. In London, 26-year-old Venetia Stanley - aristocratic, clever, bored, reckless - is having a love affair with the Prime Minister, H. H. Asquith, a man more than twice her age. He writes to her obsessively, sharing the most sensitive matters of state. As Asquith reluctantly leads the country into war with Germany, a young intelligence officer is assigned to investigate a leak of top secret documents - and suddenly what was a sexual intrigue becomes a matter of national security that will alter the course of political history.