Christopher Tolkien on The Silmarillion's development 

A recommended presentation of JRR's several Silmarillions (!) and how they stand in relationship with The Hobbit and The Lord of the Rings.
JRR's son Christopher was a creative confidant and posthumously composed his father's work and published The Silmarillion in 1977.
Source: J.R.R.T.: A Film Portrait of J.R.R. Tolkien (1996). All rights belong to Landseer and other concerned parties