Complex sentence: Many people know him best for his art based on Terry Pratchett's "Discworld", which has been included as the sleeve covers since Pratchett's other illustrator, Josh Kirby, died in 2001.
Split into simpler sentences: Many people know him best for his art based on Terry Pratchett's "Discworld". He has been included on the sleeve covers since Pratchett's original illustrator, Josh Kirby, died in 2001.