Joseph Delaney, the British fantasy author known for The Last Apprentice series of young adult novels, has died at 77, the BBC reports.

Delaney worked as a teacher before publishing his first fiction under the pen name J.K. Haderack. In 2004, he published the novel The Spook’s Apprentice in the U.K.; the following year, it was released in the U.S. as The Last Apprentice: Revenge of the Witch.

A dozen more books in the series would follow, including Curse of the Bane, Night of the Soul Stealer, and Grimalkin the Witch Assassin. Revenge of the Witch was adapted into a 2014 film, Seventh Son, by director Sergei Bodrov; its cast included Kit Harrington, Jeff Bridges, and Julianne Moore.

Delaney was the author of two other series of novels, Arena 13 and Aberrations.

His readers and admirers paid tribute to him on social media. On Twitter, author William Hussey wrote, “I’ve just learned that the wonderful Joseph Delaney of SPOOK’S APPRENTICE fame has passed away. One of the kindest, gentlest souls, and a simply phenomenal writer. He was so kind to me when I started out as a children’s author all those years ago. Sleep well, dear Joe xx.”

Author and illustrator Thomas Taylor tweeted, “I’m shocked to hear of the death of Joseph Delaney. His brilliant ’Spook’ books were a huge inspiration to me. RIP.”

Michael Schaub, a journalist and regular contributor to NPR, lives near Austin, Texas.