The great poet Francis Croft was mad. He drifted between exquisite poetic vision and being overtaken by internal daemons that at times drove him to being either homicidal or suicidal. Harvey Lawson, an Egyptologist, met Francis a few years after World War I at a house party. At a subsequent meeting Francis was in a psychotic state, crying and clawing at his face which he claimed was not his own. The two men developed an intense friendship dominated by Francis’s mental illness, and Harvey became Francis’s protector and carer.

And if he is mad, it is because one man’s brain cannot contain all the emotions and ideas and visions that are filling his without sometimes weakening and breaking down. But he will be perfectly well again, his generally well. When his is not he is in despair and when his is fit he dreads the return of his illness. What can that be like to live with?

The Bird of Night is about love and madness. It is a bleak and tender story narrated in the first person in a journal style by Harvey, now an octogenarian, reminiscing about his relationship with Francis to whom he gave himself over. We know from the beginning how the story ends and there is a deep sadness in his narration that explores the space between genius and madness and the minutiae of how it impacted their relationship.

But understanding was not control. If Francis knew what he was, he could not alter it, he had no power at all over the vagaries and eruptions of his own mind. He was helpless in the face of an attack of insanity, no matter which way it went with him, whether he was depressed or violent, whether he was hysterical, agitated or deluded by visions and voices.

Hill’s book is a beautifully written portrayal of the effects of mental illness and the experience and anguish felt by those who care about a person who suffers. She draws a detailed and exquisite portrait of loyalty, tenderness and the intense disquiet of living with mental illness. The novels sense of place is evocative and the descriptions of the countryside and wildlife provide intermittent relief from the poignancy of the story.

But the cycle of Francis’s madness was never a regular or predictable one. I had prepared myself for days, perhaps weeks, spent closeted in that dismal flat by candlelight, having to comfort and support him through his deepest apathy and depression. Certainly, for the next two days he stayed in bed or sat slouched in a chair looking as though he were half drugged, his eyes blank and all his attention turned inward upon himself. He hardly spoke to me and when he did answer a persistent question, it was with a monosyllable. He would not shave or eat or read, but only sat up once in a while and muttered to his own hands. “It’s all wrong, I tell you, it’s all wrong.” Once I caught him staring at himself in a mirror, his face very close to the glass. He looked puzzled. “I’m afraid we have not been introduced,” he said to his reflection. “I do not know your face. Should I know your face? Is this a good party?

The Bird of Night won the Whitbread Award in 1972 and was shortlisted for the Booker Prize the same year. It is the first time I have read a novel by Susan Hill, but I will definitely read more of her work.