Simplify the following text for a Grade 7 native English-speaking person.
Original: Morley 's Plaine and Easie Introduction to Practicall Musicke ( published 1597 ) remained popular for almost two hundred years after its author 's death , and remains an important reference for information about sixteenth century composition and performance .
Simplified: Morley wrote a book about music theory called Plaine and Easie Introduction to Practicall Musicke . It was published in 1597 and was read by many people for two centuries after his death .