"X" by Sue Grafton is the first novel in the Alphabet Series in which the letter in the title is not the first initial of a key word. According to the author's website, the X in the title may refer to the Roman numeral 10, a mistake or a kiss.
In a 2013 interview, Grafton said that she was considering making the X in the title of the book stand for xenophobe or xenophobia. However, when the novel was announced in early 2015, the title was given only as "X." As of 2015, Grafton has written 24 books in the "Alphabet Series," one for each letter of the alphabet through X.