Sally Rooney’s Normal People has taken the top prize at the British book awards, beating Michelle Obama’s widely-tipped autobiography Becoming and last year’s Man Booker winner Anna Burns’s Milkman, to win the title of book of the year.

The 28-year-old Irish novelist’s second book, which charts the protracted on-off love affair between two young people over several years, landed the top prize at the awards, also known as the Nibbies or “Baftas of the book trade” on Monday night. While Rooney’s novel has been nominated for a number of awards, including the Man Booker, Folio and Dylan Thomas prizes, it has only picked up a couple of titles, including the Costa novel award and Waterstone’s book of the year.

Receiving the award, Rooney called it “an enormous privilege and an honour”, thanking librarians and booksellers in particular for their help finding her audience. “I do feel astonishingly lucky,” she said.

Brett Wolstencroft, judge and manager of Daunt books, said that Rooney’s novel achieves “that rare thing, a sublime work of literary fiction that exquisitely renders a universal experience: being young, finding love, friendship and, ultimately, a sense of self”.

Because Obama’s biography sold 253% more copies than Rooney’s novel – 654,000 in all formats, compared to 185,000 – while breaking all-time audiobook sales records in the UK, it was anticipated that Becoming would be a shoo-in for an award usually hooked to sales. But Alice O’Keeffe, chair of the judges and books editor of the Bookseller said that despite Obama’s impressive sales, Normal People was the most deserving winner.

“It was a really difficult decision and we went back and forth for a good while, but after much discussion we felt that Sally Rooney is such a major talent and that her ‘difficult second novel’ was just as impressive as her debut was astonishing. She has been described as a millennial writer with millennial concerns, but I know readers in their seventies who loved Normal People. The passion that came through on the grassroots for this book is really something,” she said.

Taking factors like sales into account in the judging process meant that the Nibbies have been “accused of following everything else”, O’Keeffe said, but she felt Rooney’s status as a bestseller in literary fiction, a genre that has struggled with sales in recent years, had yet to be acknowledged.

“She has transcended literary fiction. It is not something I can say about many young writers,” she said.

Obama’s memoirs, which trace her trajectory from growing up in south side Chicago to the White House, still picked up two British book awards, for best non-fiction and audiobook of the year. Obama’s performance of her own memoir beat Ben Whishaw’s telling of the late Stephen Hawking’s final book, Brief Answers to the Big Questions, and Robert Galbraith’s Lethal White, narrated by Robert Glenister.

In other categories, three-time winner David Walliams has picked up the award for children’s book of the year again, this time for The Ice Monster, the story of a 10-year old Victorian orphan and a 10,000 year old woolly mammoth. Sam Taylor’s translation of Leïla Slimani’s Lullaby picked up the gong for best debut novel, while Bosh!, a cookbook written by “vegan Jamie Olivers” Henry Firth and Ian Theasby received the award in the lifestyle category, seeing off household names Tom Kerridge and Yotam Ottolenghi.