Brad East in The Hedgehog Review:
The conceit at the heart of Mick Herron’s Slow Horses novels is simple. There is a house in London for misfit spies. When MI5 is unable, for one reason or another, to fire failed employees, it opts to send them there. The exile is permanent, though the losers who suffer it do their best to pretend it isn’t. It’s a win-win for the service, in any case. No one gets sued. HR is pacified. And banishment proves either so unbearably dull and humiliating that the misfit spies voluntarily quit, or they remain there forever, whiling away the hours without hope of redemption. It is said of the souls in Dante’s purgatorio that the unhappiest are happier than the happiest on earth. Conversely, the happiest in Herron’s inferno are unhappier than the unhappiest outside its walls.
More here.