Even actuallier, the first book was titled "The World of Ā" but "to reduce printing costs, the 1953 and 1964 Ace Books paperback editions were entitled The World of Null-A, and the symbol Ā was replaced with "null-A" throughout the text." [1]

[1] https://en.wikipedia.org/wiki/The_World_of_Null-A#Publicatio...