Clare Dunkel, who wrote thrillers under the pen name Mo Hayder, has died at 59, The Guardian reports. The cause of death was motor neuron disease.

Dunkel made her literary debut in 1999 with the novel Birdman, about a London detective on the trail of a mass murderer. The novel quickly became a bestseller; a reviewer for Kirkus called it “graphic, disturbing, splendidly readable.” A sequel, The Treatment, followed two years later.

Her fifth novel, Ritual, launched her Walking Man series; the fifth book in that series, Wolf, was her most recent, published in 2014. Before her death, she completed work on The Book of Sand, which will be published by Century in the U.K. early next year under the pen name Theo Clare.

Dunkel’s admirers paid tribute to her on social media. Author Val McDermid wrote, “Deeply saddened to hear of the death of Mo Hayder. Mo was a genuine original, who pushed at the limits of what the genre could do. To lose her so young to such a horrible disease fills me with sorrow, not least for the books she had in her head that we'll never read.”

And writer Simon Kernick tweeted, “I’m really shocked and saddened to hear about the death of Mo Hayder at the age of 59. She was an amazing and original writer, and a lovely person as well. I met her a number of times in the early part of my career and she was always great company. She will be sorely missed.”

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