X not Ex. He wrote two X and Y.
From
https://mysteriousuniverse.org/2019/11/of-x-y-and-z-the-search-for-the-lost-works-of-charles-fort/
There had in fact been the accumulation of writings in the form of not one, but two books written by Fort, given the Spartan titles of X and Y, which were never published, despite the efforts of Theodore Dreiser, who had been particularly impressed by them.
X was, as Fort explained, based on a concept which had been inspired by the oddities he had been collecting during his descents into the catacombs within the pages of tomes kept at the New York Public Library. X was, however, not the exposition on uncanny things that The Book of the Damned would ultimately be: it was instead a fictional work which borrowed such ideas, and attempted to unite them with an underlying meaning.
In Fort’s own words, as explained in a letter to Dreiser:
As noted by Fort’s biography Jim Steinmeyer, “Fort’s letter to Dreiser about X included a number of puzzling statements. The finished text was nearly a hundred thousand words, but Fort seemed uninterested in boo publishing, thinking that it might make a better series in a magazine.” This had not been the most curious thing Fort would express to Dreiser about X, however. Fort seemed to hint at the idea that the “fictional” work was based in what, at very least to Charles Fort, might very well have appeared to be fact.
“I’ve given up fiction,” Fort told Dreiser. “Or in a way I haven’t. I am convinced that everything is fiction; so here I am in the same old line.”
Steinmeyer argues that, “There’s little question that Fort took X seriously, and the doubts he expressed were his modest way of kidding his efforts.” After all, the same general themes present in surviving descriptions of X would appear to have been carried over into The Book of the Damned. Following the destruction of the X and Y manuscripts, Fort went back to collecting notes on oddities:
