A spokeswoman for Hart's husband, advertising mogul Lord Maurice Saatchi, 64, said she died in London on Thursday after battling cancer.
Her agent Ed Victor said for two years she kept secret that she was suffering from cancer.
"She wrote one of the most stunning, dazzling, moving, powerful first novels of the last half-century," Victor said.
"Her passion for literature and her passion for poetry burned with the purest flame."
Hart had been due to present a series of poetry readings at the Donmar Warehouse in London's Covent Garden this week.
Days before her death, she endorsed a campaign against illiteracy led by the London Evening Standard newspaper.
In a statement dictated from her bed, she said: "Without reading, and for me especially poetry, I would have found life less comprehensible, less bearable, and infinitely less enjoyable."
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Her agent Ed Victor said for two years she kept secret that she was suffering from cancer.
"She wrote one of the most stunning, dazzling, moving, powerful first novels of the last half-century," Victor said.
"Her passion for literature and her passion for poetry burned with the purest flame."
Hart had been due to present a series of poetry readings at the Donmar Warehouse in London's Covent Garden this week.
Days before her death, she endorsed a campaign against illiteracy led by the London Evening Standard newspaper.
In a statement dictated from her bed, she said: "Without reading, and for me especially poetry, I would have found life less comprehensible, less bearable, and infinitely less enjoyable."
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