Z for Zachariah

In Z FOR ZACHARIAH by Robert C. Obrien, nuclear war has devastated America, except for the valley where Ann Burden has been living alone for over a year - until Loomis, a scientist in a radiation-proof suit, arrives. She hopes they will be companions but his behaviour towards her becomes increasingly threatening as he attacks her and then cuts off her food supply and tries to bring her under his control. Although there may be no one else alive, Ann steals his suit and leaves the valley in search of humanity.