I didn't know about this novella* until very recently, when I read and reviewed Karin Slaughter's latest novel, Criminal. It's described as #6 in Slaughter's Will Trent series, although given its length and its tangential relationship to the rest of the series, it would probably be more accurate to think of it as #5.5.
The action takes place over a day, at Hartsfield–Jackson Atlanta International Airport, where Georgia Bureau of Investigation agent Will Trent has been given an unpleasant assignment, supposedly for defying his boss. He becomes convinced that a child has been abducted and from that moment the hunt is on for the child and the perpetrator.
The work is a good example of Karin Slaughter's skill. While very short (readers should know that about 40% of the e-book consists of an extract from Criminal), it's tightly plotted and suspenseful. All in all, this is a welcome gift for fans of the Will Trent series. In addition, it's not a bad way for readers to find out if they like Karin Slaughter's writing. If they do, it would probably be best to start with Triptych.**
*More of a long short story, really.
**I'm sure that some readers would recommend going back to the very beginning with Blindsighted, the first book in Slaughter's earlier (but related) Grant County series.