Dead Lions (Slough House #2), Mick Herron

From the Intelligence Service purgatory that is Slough House, where disgraced spies are sent to see out the dregs of their careers, Jackson Lamb is on his way to Oxford, where a former spook has turned up dead on a bus. Dickie Bow was a talented streetwalker once, good at following people and bringing home their secrets. He was in Berlin with Lamb, back in the day.

But he's not an obvious target for assassination in the here and now. On Dickie's phone Lamb finds the last message he ever left, which hints that an old-time Moscow-style op is being run in the Intelligence Service's back-yard. Once a spook, always a spook, and even being dead doesn't mean you can't uncover secrets.