story identification - Teenage girl in life-sustaining valley on poisoned Earth visited by another survivor

I'm trying to identify a book I read in school, though I can't recall at what grade level; let's say it had to have been published before I graduated high school in 2000. Despite being required reading, it didn't feel much like a classic.

The story took place on Earth, after some catastrophe had caused the air to become poisonous. However, one teenage girl was able to survive because her house was in a valley which was somehow protected from the poisonous winds and was able to sustain its own ecosystem. One day, a man wearing a protective full-body suit and gas mask appears in the valley, and both are rather surprised to find someone else alive. She feeds and shelters the man and shows him how she had come to survive in the valley. Rather abruptly…

the man attempts to rape her, but she overpowers him somehow. She steals his protective suit and starts to leave the valley. He tells her that he had seen birds in a certain direction away from the valley, indicating there might be life there as well, and she leaves.

I believe there was an author's note either at the beginning or end of the book stating that the author had actually died while writing the manuscript, and that their friends or family had finished the book by going off the author's original notes, which I suppose may explain the abruptness of the ending.

In case it wasn't clear from the above, I didn't really like the book. But I'm curious about it regardless.

Answer

"Z for Zachariah" by Robert C. O'Brien.

From Wikipedia:

Ann Burden is a teenage girl who believes she is the last survivor of a nuclear war. Since her family's disappearance on a search expedition, she has lived alone on her farm in a small valley spared from radiation poisoning. A year after the war, a stranger in a radiation-proof suit approaches her valley. Afraid he might be dangerous, Ann hides in a cave and does not warn the man when he mistakenly bathes in a radioactive stream. When he falls ill, her fear of being alone forever leads her to reveal herself to help him. She discovers that the stranger is John Loomis, a chemist who helped design a prototype radiation-proof "safe-suit" at an underground lab near Ithaca, New York. Ann moves him into her house and fantasizes about eventually marrying him.

As Loomis becomes more ill and delirious, he has traumatic flashbacks to the underground lab, and talks of how he shot his coworker, Edward, who tried to take the safe-suit to find his family. Though troubled by this revelation, Ann continues to nurse him through his illness and keeps secret her knowledge of Edward's death. As Loomis recovers, Ann is taken aback when he begins criticizing her farming and giving her orders. He forbids her to touch the safe-suit. He orders her to plant wheat and beets to preserve seed stock, and though she acknowledges this is sensible, his explanation that they have to plan "as if this valley is the whole world and we are starting a colony," makes her uneasy. Her uneasiness increases when she asks if he was ever married, and he grabs her hand roughly. He demands to know why she asked and refuses to let go of her until she answers, pulling her off balance until she falls and inadvertently hits his face. He rebukes her for this and does not speak of the incident again. One night soon afterwards, she awakes to hear Loomis in her room. When he attempts to rape her, she flees to the cave again.