The Secret Hours, by Mick Herron

There’s not a tardy nag in sight in this Slow Horses-adjacent tale that offers the perspective of long-suffering civil servants who support the parliament. The Monochrome inquiry into misconduct of the British secret services has been set up by politicians and their apparatchiks to spike the guns of MI5’s First Desk (she goes unnamed but it’s clearly Diana Taverner). She has found a way to make it totally ineffectual, wasting the time, lives and careers of everyone involved. That is until the Otis file is leaked to the inquiry and the first genuine witness is interviewed after two years of operation. Monochrome is quickly shut down, but the civil servants involved decide not leave any loose ends and hear the witness out. Her tale reveals an off-the books operation in post-wall Berlin and its lingering effects to the present day. Although the protagonists are not identified by their real names, it becomes clear that this is the story of how Jackson Lamb came to run Slough House. It fills in the details of several backgrounds and connections along the way, leading to a very satisfying resolution that provides both justice (or revenge) and understanding on a number of levels.