As a child of the 1980s, I was of course convinced that nuclear war was imminent. Along with my over-anxious friends, I ducked under tables if lightning flashed, looked out for mushroom clouds on the horizon, and once mistook the hum of a lawnmower for a three-minute warning siren. It sounds ridiculous now, but at the time I wasn’t the only one going on CND marches with well-meaning parents, watching terrifying post-apocalyptic TV dramas from behind the sofa and reading kids’ books about armageddon.

For me, the most devastating response to imminent nuclear catastrophe remains Raymond Briggs’ 1982 graphic novel When the Wind Blows, in which a sweet-faced elderly couple make the best of things after the blast. They do as they’re told, constructing a fallout shelter from doors in line with the Protect and Survive pamphlet, and try to muddle through while the gently crayoned panels get progressively darker and any hope of survival dwindles.

But Robert C O’Brien’s Z for Zachariah, first published in 1974, is an end-of-the-world novel fuelled by the power of hope – hope, which comes to light, in the Pandora myth, after the darkest of forces are unleashed. It’s written as the diary of 16-year-old Ann Burden, the daughter of American farmers who find in the wake of nuclear war that their remote valley is sheltered from radiation by its own weather system. Nearly a year has passed since Ann’s family and neighbours set out looking for other survivors, and never returned; Ann has listened as radio broadcasts fall silent, and feels “pretty sure I was the only person left in the world”. Practical and determined, she’s been raiding the local store for supplies, tending the crops and animals on the smallholding, careful to take water only from the stream that bubbles up from beneath the valley and not the one that flows in from the deadness outside. She even puts flowers in the tiny church.

As the book opens, she sees what she dreamed about in the months after the war ended: signs of life from the outside world, a column of smoke getting nearer. Torn between joy and fear, she retreats to a cave in the side of the valley – but also puts on her smart trousers, for visitors: the habits of normality die hard. The stranger turns out to be a man in a radiation suit, dragging a cart of supplies, who whoops in ecstasy at finding this oasis before taking off his suit and bathing – in the wrong stream.

Ann’s initial reluctance to reveal her presence to the last man in the world (if A is for Adam, Z is for Zachariah) is set out in simple, unanswerable terms. “This man is a stranger, and bigger and stronger than I am. If he is kind, then I am all right. But if he is not – what then? He can do whatever he likes, and I will be a slave for the rest of my life.” Mr Loomis doesn’t seem at first like the figure of nightmares familiar from the post-apocalyptic fictions of Margaret Atwood and Cormac McCarthy, or Helen Simpson’s short story Diary of an Interesting Year – the neo-savage who has thrown off his humanity along with the other constraints of civilisation. But as Ann nurses him through the radiation sickness he contracts from the stream, and as he takes charge in the valley that once belonged to Ann alone and decides what form their relationship should take, she slowly comes to the realisation that “whatever Mr Loomis was planning, at the end of the plan was a picture, and it was of me, tied up in the house”.

So where’s the hope in this terrifying scenario, which is as chilling in existential terms as it is in the arena of sexual politics? It comes in the enduring natural world of the valley and the figure of Ann: her bravery, her fortitude, but most of all the way she combines resistance with generosity to the interloper in her Eden. She dares to hope he can conquer his worst impulses: while he is tying up the animals and putting a lock on the store, she is still prepared to share the fruits of the valley with him – if he’ll just leave her alone. Mr Loomis rubbishes the idea of wasting petrol on a trip to the library in the next town, and disapproves of Ann spending time in the church when she could be working the fields. He is so focused on survival, he doesn’t stop to consider what one might use it for. That’s why Ann has to be the one who leaves: of the two of them, she’s the one who is emotionally and imaginatively strong enough to keep her hope alive.

This is a hard ending for a young reader to accept – and Ann herself struggles with the “childishness” of her need for fairness, in a book that is fascinated, like much of the best YA, with the threshold between childhood and adulthood (her formal references to “Mr Loomis” throughout signal the age difference between them, as well as her need to keep him at arm’s length). Decades later, though, Ann’s decision to step from her bounded Eden into the ominous wasteland outside reads as a testament to the power of choosing hope over surrender.