The Secret Hours by Mick Herron

A gripping spy thriller from the bestselling author of Slow Horses, about a disastrous MI5 mission in Cold War Berlin—an absolute must-read for Slough House fans.
Set in the MI5 world of Slow Horses, now an Apple Original series from Apple TV+ starring Gary Oldman and Kristin Scott Thomas.
Two years ago, a hostile prime minister launched the Monochrome inquiry, investigating “historical over-reaching” by the British Secret Service. Monochrome’s mission was to ferret out any hint of misconduct by any MI5 officer—and allowed Griselda Fleet and Malcolm Kyle, the two civil servants seconded to the project, unfettered access to any and all confidential information in the Service archives in order to do so.
But MI5’s formidable First Desk did not become Britain’s top spy by accident, and she has successfully thwarted the inquiry at every turn. Now the administration that created Monochrome has been ousted, the investigation is a total bust—and Griselda and Malcolm are stuck watching as their career prospects are washed away by the pounding London rain.
Until the eve of Monochrome’s shuttering, when an MI5 case file appears without explanation. It is the buried history of a classified operation in 1994 Berlin—an operation that ended in tragedy and scandal, whose cover-up has rewritten thirty years of Service history.
The Secret Hours is a dazzling entry point into Mick Herron’s body of work, a standalone spy thriller that is at once unnerving, poignant, and laugh-out-loud funny. It is also the breathtaking secret history that Slough House fans have been waiting for.