The History of Middle-earth
In earlier versions of the legendarium, the name Ilúvatar meant "Father for Always" (in "The Book of Lost Tales", published as the first two volumes of "The History of Middle-earth"), then "Sky-father", but these etymologies were dropped in favour of the newer meaning in later revisions. Ilúvatar was also the only name of God used in earlier versions – the name Eru first appeared in "The Annals of Aman", published in "Morgoth's Ring", the tenth volume of "The History of Middle-earth".