Author of the popular science-fiction book series Discworld Sir Terry Pratchett has today lost his fight for life at the age of 66. It was reported he died with his family around him of early-onset Alzheimer’s.

Pratchett was most well known for his series of books known collectively as the Discworld books, about a disc-shaped world set on the backs of four elephants positioned on the back of a giant turtle called Great A’Tuin. The series while it dealt with many fantasy sub-genres such as fairy tales and horror also contained a lot of examination of big issues such as the meaning of death, religion and business and politics, with the storylines of the main characters often interweaving between books. Forty Discworld novels were published in Pratchett’s lifetime, as well as certain books being adapted into other mediums such as graphics novels and televisual programming.

Pratchett announced his suffering from early-onset Alzheimer’s back in 2007, but continued to write anyway, completing a forty-first Discw0rld novel just last year. He used his fame to campaign for assisted dying and to raise awareness of dementia and other related illnesses.

The news was announced over Sir Terry’s Facebook page with a statement that read: “It is with immeasurable sadness that we announce that author Sir Terry Pratchett has died. The world has lost one of its brightest, sharpest minds. Rest in peace Sir Terry Pratchett.”

Pratchett’s daughter Rhianna Pratchett has been tweeting out the news too and thanked fans for their well-wishes to herself and Pratchett’s family at this time.

Many thanks for all the kind words about my dad. Those last few tweets were sent with shaking hands and tear-filled eyes.

— Rhianna Pratchett (@rhipratchett) March 12, 2015