The above was posted on Twitter by Jenna, @joaktree33, who writes, "Robert Anton Wilson & William S. Burroughs are often credited as the originators of the “23 enigma”, but there’s an even earlier reference to the mysterious number in the Sept. 1952 Black Magic comic book by Jack Kirby." And please follow the link for another posted page.
And in fact, interest in 23 goes back well before the 1950s, as the Illuminatus! trilogy notes. See this interesting Wikipedia piece on the phrase "23 skidoo."
As the piece notes, a dire 23 dates back to Charles Dickens' classic 1859 novel, A Tale of Two Cities. At the end of the novel, poor Sydney Carton is No. 23 in line to the guillotine, as I verified by looking up the text at Project Gutenberg. (Fun fact: Sydney Carton was once portrayed on radio by Orson Welles.)