Terry Pratchett, the bestselling author known for his fantasy series “Discworld,” has died at the age of 66, his publisher announced this morning.

Pratchett was diagnosed almost eight years ago with a rare form of Alzheimer’s disease. The voluminous Discworld series has sold over 80 millions copies and has been translated into 37 languages worldwide. He published his last novel in Summer 2014.

Pratchett's work was often labelled as comic fantasy. But some argue that the depth of his books has been largely undershadowed by their humor.

With knowledge of his declining condition, fellow novelist and colleague Neil Gaiman defended Pratchett, saying that a serious anger is what drove much of his writing:

“The anger is always there, an engine that drives. By the time Terry learned he had a rare, early onset form of Alzheimer’s, the targets of his fury changed: he was angry with his brain and his genetics and, more than these, furious at a country that would not permit him (or others in a similarly intolerable situation) to choose the manner and the time of their passing.”

“And that anger, it seems to me, is about Terry’s underlying sense of what is fair and what is not. It is that sense of fairness that underlies Terry’s work and his writing, and it’s what drove him from school to journalism to the press office of the SouthWestern Electricity Board to the position of being one of the best-loved and bestselling writers in the world.”

“He will rage, as he leaves, against so many things: stupidity, injustice, human foolishness and shortsightedness, not just the dying of the light. And, hand in hand with the anger, like an angel and a demon walking into the sunset, there is love: for human beings, in all our fallibility; for treasured objects; for stories; and ultimately and in all things, love for human dignity.”

Read Gaiman's full profile of Pratchett on The Guardian.