Simplify the following text for a Grade 7 native English-speaking person.
Original: The Silmarillion is a collection of J. R. R. Tolkien 's mythopoeic works , edited and published posthumously by his son Christopher Tolkien in 1977 , with assistance from Guy Gavriel Kay , who later became a noted fantasy writer .
Simplified: The Silmarillion is a book written by J. R. R. Tolkien . It was first published posthumously in 1977 , edited by his son Christopher Tolkien .