The William Hill Sports Book of the Year Award has been shared for the first time in its 30-year history with A Boy in the Water, Tom Gregory’s charming coming-of-age story about being the youngest person to swim the Channel, sharing the £30,000 prize with The Lost Soul of Eamonn Magee, an unflinching biography of the Belfast boxer by Paul D Gibson.

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It was thought Tiger Woods by Jeff Benedict and The Boy on the Shed by Paul Ferris were the frontrunners but the judges, who in recent years have plumped for left-field winners – including on doping in 1960s racing, the dark side of Australian rules football, surfing, the meaning of English football and a photo essay on the cyclist Tom Simpson – decided to throw another curve ball.

The co-founder of the award, Graham Sharpe said six judges had found it impossible to separate Gregory and Gibson’s books despite heated debate that went on long past midnight. “In the 30 years since launching the award we have occasionally considered, but never ultimately awarded, a dead heat,” he said. “But the judges found it impossible to separate these two jointly deserving but very different books.”

Gregory’s story, which is told over 180 taut pages, tells of how he came to swim the Channel aged 11 years and 336 days, which makes him still the youngest person in history to achieve the feat. Despite wearing no wetsuit, surviving on cans of tomato soup and chocolate biscuits, and occasionally hallucinating, falling asleep and crying so much his goggles filled with water, he finished his 32-mile swim in 11hr 54min.

But Gregory’s book is also a paean to a forgotten age, of travelling around the country with his inspirational coach, John Bullett, and several other youngsters from an estate in south-east London, many of whom became long-distance swimming sensations.

Gibson’s book tells of the story of Magee – a hugely talented fighter who drank, gambled, suffered depression and had several brushes with the law and the IRA. It is an astonishing and brutal story, but with enough laughter and love to break through the gloom.

Donald McRae, the Guardian writer and a two-times winner of the award, described it as “an unforgettable book”, adding: “Eamonn Magee has lived a harrowing life. But it is a life that gains fresh meaning in this raw, riveting book. Magee thought he had run out of luck years ago but, this time, he got lucky again. In Gibson he has found a writer who has the grit and the tenderness, the skill and the heart, to lay Magee’s life bare on the page.”

The other shortlisted books were Fear and Loathing on the Oche by King Adz, Berlin 1936: Sixteen Days in August by Oliver Hilmes and Sevens Heaven: The Beautiful Chaos of Fiji’s Olympic Dream by Ben Ryan.