With the artist Jackie Morris, Macfarlane published "The Lost Words: A Spell Book" in October 2017. The book became what the Guardian called 'a cultural phenomenon', winning Children's Book of the Year at the British Book Awards jointly with "The Hate U Give" by Angie Thomas. The "lost" words of the book's title are twenty of the names for everyday nature—from "Acorn" through to "Wren" by way of "Bluebell", "Kingfisher", "Lark" and "Otter"—that were controversially dropped from inclusion in the Oxford Junior Dictionary due to under-use by children. Grassroots campaigns sprang up to raise money to place copies of the book in every primary and special school in all of Scotland, half of England and a quarter of Wales.