As explained in The History of Middle-earth, Christopher drew upon numerous sources for his narrative, relying on post-Lord of the Rings works where possible, but ultimately reaching back as far as the 1917 Book of Lost Tales to fill in portions of the narrative which his father had planned to write but never addressed.
Som forklaret i The History of Middle-earth, trak Christopher på en lang række kilder til sin fortælling, beroede på værker før Ringenes Herre, hvor det var muligt, og endte ultimativt med at nå så langt tilbage som til Book of Lost Tales fra 1917 for at udfylde de dele af fortællingen, hans far havde planlagt at skrive, men som aldrig blev adresseret.