My argument is based on the article in the OP.
From the article:
"Written over half a century ago, it is [Atlas Shrugged] clearly the greatest and most influential novel of the 20th century."
Did you catch that little word, novel, in there, by any chance?
Relax, Francis.
My original post was meant for Patrick1, but my stupid fingers sent it to you by mistake.
Besides, I think that having the title of second-most influential book is even more conclusive in regards to the prolific nature of “Atlas”. You’re covering all genres of the written word, not just novels.