X
Besides the 24th letter in the alphabet, X stands for many things - the number ten, an unknown quantity, a mistake, a cross, a kiss...
It's definitely a ten out of ten but perhaps it's also Sue Grafton’s darkest and most chilling novel too. X features a remorseless serial killer who leaves no trace of his crimes. Once again breaking the rules and establishing new paths, Grafton wastes little time identifying this deadly sociopath. The test is whether private investigator Kinsey Millhone can prove her case against him—before she becomes his next victim.
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Sue Grafton
In Sue Grafton's bestselling mystery series, hard-boiled detective Kinsey Millhone is an ex-cop and rebellious loner. Now a private investigator, she solves all the most difficult cases in (fictional) Santa Theresa, California.
In her 30s, Millhone ages one year every 2 1/2 books, from 32 in A to 38 in W. http://www.suegrafton.com/bookshelf.php
Sue Grafton died in December 2017, and her daughter said in the death announcement that Sue had not wanted a ghost writer to continue the series, so the series ends with book 25, Y is for Yesterday.
Kinsey Millhone
Kinsey Millhone consists of twenty-five books. The current recommended reading order for the series is provided below.

