Impossible Creatures by Katherine Rundell has been named the 2023 Waterstones book of the year.

The children’s novel, about a magical archipelago where all mythical creatures still reside, was voted for by booksellers as the book they most enjoyed recommending to readers over the past year.

Rundell said that she was “truly, utterly thrilled” on hearing the news. “I did not believe it until they showed me it in writing. I made my editor show me written-out proof.”

Alongside Impossible Creatures, In Memoriam by Alice Winn was named Waterstones novel of the year, and Murdle by GT Karber was named Waterstones gift of the year. Winn also won the Waterstones debut fiction prize in August.

Impossible Creatures is “as close to perfect as fiction gets: immaculate world-building, dazzling storytelling, and adventure galore,” said Bea Carvalho, head of books at Waterstones. “It is an immediate classic which children will delight in for years to come, and which will remind adults of the genius to be found within the pages of children’s books.”

Rundell triumphed over 13 other shortlisted authors, including Zadie Smith with her historical novel The Fraud and Ann Patchett for Tom Lake.

Rundell has written several books for children, including Rooftoppers, which won the Waterstones children’s book prize and the Blue Peter book award for best story in 2014. She has also written three non-fiction books for adults, including Super-Infinite: The Transformations of John Donne, which won last year’s Baillie Gifford prize, and The Golden Mole and Other Living Treasure, which was shortlisted for last year’s Waterstones book of the year.

“The fact that this book has won rather than any of my others is so thrilling because this, in being a children’s book that I hope could also be read by adults, is the book that I hope could speak to everyone,” said Rundell. “It has the best of everything that I’ve learned and that I’ve read and all of the scholarship I’ve come across in the last 15 years of my working life, and a kind of distillation of everything I know and hope.”

She added that children’s books are a “way of talking more broadly about the things we value and the things we love” and that they are “philosophy’s more brightly painted cousin”.

Rundell first sent an email to her agent about the book in 2016, and then spent “really joyful hundreds of hours in libraries reading about these mythical creatures”.

She said that she is currently working on the second book of the Impossible Creatures series, of which there will be at least three volumes.

In 2022, The Story of Art Without Men by Katy Hessel was named book of the year. Previous winning titles include Hamnet by Maggie O’Farrell, Stoner by John Williams and Normal People by Sally Rooney.

• Impossible Creatures by Katherine Rundell (Bloomsbury Publishing PLC, £14.99). To support the Guardian and Observer, order your copy at guardianbookshop.com. Delivery charges may apply.