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 . 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 .