Sam McBratney, the prolific author whose Guess How Much I Love You is considered one of the most enduring and beloved children’s books of the 20th century, died on Sept. 18. He was 77.

His death, which was announced in a news release by his American publisher, Candlewick Press, comes just days before the sequel to Guess How Much I Love You, titled Will You Be My Friend?, is set to be published.

McBratney was born in Northern Ireland and worked as a schoolteacher for two decades before pursuing writing full-time. His first books were mostly young adult novels, but his career took off when he turned his attention to illustrated books for children.

He published Guess How Much I Love You in 1994. The story of two hares who describe their affection for each other, it quickly became a bestseller, and one of the lines from the book, “I love you to the moon and back,” has become one of the best-known in children’s literature. The book has sold over 50 million copies across the world and spawned several companion books, including When I'm Big: A Guess How Much I Love You Storybook and Guess How Much I Love You: Colors.

The sequel, Will You Be My Friend?, is slated for publication on Sept. 29. A reviewer for Kirkus praised the book, writing, “Readers are likely to love it to the moon and back.”

Michael Schaub is a Texas-based journalist and regular contributor to NPR.