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...

Ah, thanks for mentioning that. If you get the chance, the other two are pretty great as well. As an aside, one of my favorite authors, Neil Asher, recently blogged about A.E. van Vogt:

http://theskinner.blogspot.com/2020/06/destination-universe-...