I don't see why there should be a problem.
I think HarryT's claim that it could "technically" be considered a derived work is rather farfetched, perhaps a textbook example of a law being interpreted in a truly ludicrously literal manner.
I would be willing to bet (a lot) that any judge,
ANY, would simply laugh such a case out of court.
Prosecutor: "Your honor, the accused stands charged of creating a derived work by substituting every letter in this book with a different random letter."
Judge: "WTF?
"
Prosecutor: "The letter count of the defendant's book is clearly derived from my client's book. I think this is a serious problem, don't you?"
Judge: "Get out of my court before I beat you over the head with a clue-by-four."