"X" is the transliteration of the Cyrillic character in question as per the 1995 version of ISO/R 9; but the 1968 version that was in effect when The TeXbook wa

"X" is the transliteration of the Cyrillic character in question as per the 1995 version of ISO/R 9; but the 1968 version that was in effect when The TeXbook was first published has it as "kh". See https://en.wikipedia.org/wiki/ISO_9 https://en.wikipedia.org/wiki/ISO_9 You may want to send Knuth a note that he should update this for the next printing; perhaps you'll get one of his rewards.