Nobody's Girl: A Memoir Of Surviving Abuse And Fighting For Justice, co-written with Amy Wallace, also picked up the prize for non-fiction narrative book of the year at a ceremony at Grosvenor House in London on Monday.
Giuffre, who took her own life in 2025 aged 41, was one of the most prominent accusers of paedophile financier Jeffrey Epstein, convicted sex trafficker Ghislaine Maxwell and former prince Andrew Mountbatten-Windsor.
Her widely-praised book provides a detailed account of her alleged trafficking by Epstein and Maxwell, and includes allegations against powerful men such as Mountbatten-Windsor.
The win is said to "remind us of the power of books to uphold an author's legacy", in light of the revelations of the Epstein files – the US Department of Justice's document dump relating to the convicted sex offender.
The awards ceremony also commemorated Dame Jilly Cooper with a special posthumous award.
Dame Jilly died unexpectedly in October aged 88, and was best known for her raunchy 1980s romance novel Rivals, which has seen been adapted into an award-winning Disney+ series.
Philip Jones, editor of The Bookseller and chair of the judges at the British Book Awards, said: "Our winners represent the very best of the book trade, standing up for the books and the authors when others would try to stand them down.
"The British Book Awards affirms our creatives, our entertainers and our truth-tellers, and we applaud those who did so much to bring their work to the public.
"In 2025 the importance of the book was manifest, it was a beacon to many and a heartbeat for all."
Alice O'Keeffe, head of books for The Bookseller and chairwoman of the books of the year, said the judges championed Ms Giuffre's memoir, adding: "With this award we recognise the late author's extraordinary courage and determination, supported by (publisher) Doubleday's sensitive campaign. #BelieveHer."
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