Simplify the following text for a Grade 8 native English-speaking person.
Original: In the years after his death in 1991 , several additional books have been published based on his sketches and notes ; these include Hooray for Diffendoofer Day !
Simplified: In the years after his death in 1991 , several books have been published based on his sketches and notes ; these include Hooray for Diffendoofer Day !