"X" is the twenty-fourth novel in Sue Grafton's "Alphabet" series of mystery novels and features Kinsey Millhone, a private eye based in Santa Teresa, California, a fictional version of Santa Barbara, California. The novel, set in the late 1980s, finds Kinsey pursuing a sociopathic serial killer. The novel, was published by G.P. Putnam's Sons, was released in the United States on August 25, 2015.