Since the first novel, The Colour of Magic (1983), 40 Discworld novels have been published as of November 2013. Pratchett, who suffers from Alzheimer's disease, has said that he would be happy for his daughter Rhianna to continue the series when he is no longer able to do so. The original British editions of the first 26 novels, up to Thief of Time (2001), had distinctive cover art by Josh Kirby; the American editions, published by Harper Collins, used their own cover art. Since Kirby's death in October 2001, the covers have been designed by Paul Kidby. Companion publications include eleven short stories (some only loosely related to the Discworld), four popular science books, and a number of supplementary books and reference guides.