Spook Street by Mick Herron

What happens when an old spook loses his mind? Does the Service have a retirement home for aging spies who can no longer remember their secrets are secret? Or are senile spies taken care of in a different, more permanent fashion? These are the paranoid concerns of David Cartwright, a Cold War-era operative and one-time head of MI5 who is sliding into dementia, and questions his grandson, River, must answer now that the spy who raised him sometimes forgets to wear pants. But River, himself an agent at Slough House, MI5's outpost for disgraced spies, has other things to worry about. A bomb has detonated in the middle of a busy shopping center and killed forty innocent civilians. The "Slow Horses" of Slough House must figure out who is behind this act of terror before the situation escalates.