The Twenty Eighth Book of 2026: Spook Street, by Mick Herron

Started 9/1/26, finished 9/4/26

Family has always been a part of the Slow Horses universe. There is, of course, the dysfunctional family of the slow horses themselves, who don't appear to care about each other but who act like siblings at war with one another until a common threat unites them.

But there's biological family as well, embodied by River Cartwright and his grandfather, David Cartwright, affectionately known as the "Old Bastard" or "O.B.". The O.B. spent his career high up in MI5, not as an official director but as a power behind the scenes. He and his late wife Rose raised River from the time he was seven, when his mother abandoned him, and grandfather and grandson are very close.

But dementia is rapidly eroding the O.B.'s mind, and River is afraid the O.B. will harm someone without realizing what he's doing.

This being a Slow Horse's book, of course that happens, sending River on an unsanctioned voyage to France (this is one of River's primary modes of action, going off without getting permission or even informing people of what is going on).

In the meantime, MI5 are looking for the O.B., who River hid in Catherine Standish's bedroom before leaving for France. Catherine is trying to resign from MI5 after being kidnapped in the last book, but harboring the O.B. brings her back to Jackson Lamb's attention (not that she ever was really free of it).

Back at Slough House itself, a new slow horse has appeared, P.J Coe. Coe was introduced in the short story The List,which fits into the timeline immediately before the previous book. Author Mick Herron has a way of planting seeds that take a book or two to bear fruit. Why Coe ended up a slow horse isn't clear, but it's obvious he was traumatized. 

What River Cartwright discovers in France, and the deepening of his family story, I will leave to the reader to discover. Let's just say that a whole new garden of seeds is planted, promising a rich harvest in the books to come. New characters are introduced, another slow horse is lost, and we wait for new growth to appear.