Scottish author and stand-up comic AL Kennedy has won the £25,000 Costa Book of the Year award for her novel Day.
AL Kennedy's Day features the story of a World War II veteran
Kennedy had been favourite for the former Whitbread prize before the winner was announced in London.
Historian Simon Sebag Montefiore, poet Jean Sprackland, former postwoman Catherine O'Flynn and children's author Ann Kelley were also up for the prize.
The writers each won individual categories last month and were competing for the main award.
Kennedy won top novel, Sebag Montefiore had best biography and O'Flynn's book was named top first novel. Sprackland won for poetry and Kelley for children's books.
Kennedy's Day, the story of a World War II veteran who confronts his past while working as an extra in a prisoner of war film, was hailed by the judges as a "masterpiece".
Rodney Troubridge, of Waterstone's fiction marketing, said: "If ever an award was long-deserved this is it.
"AL Kennedy has long been a favourite of Waterstone's booksellers and customers, and this win will bring her wonderful writing to a wider audience."
Sebag Montefiore was second favourite to win with his biography Young Stalin - the prequel and companion to Stalin: The Court of the Red Tsar.
What Was Lost by O'Flynn, is the story of a security guard who spots a child on CCTV 20 years after she disappeared. The book was rejected by publishers 15 times before being released and scooping the debut novel award.
Kelley's The Bower Bird follows a 12-year-old girl awaiting a heart transplant who rebels when her family moves house.
And Sprackland's third collection of poems, Tilt, describes a world in freefall.
The judging panel was chaired by best-selling author Joanna Trollope and included former Blur star Alex James, GQ editor Dylan Jones and BBC news presenter Emily Maitlis.
The ceremony took place at The InterContinental Hotel in London.