So the denizens of Slough House are sent to basically count paper clips until they quit. But action seems to keep finding them and they often prove they are, at least slightly, less incompetent than it appears on the surface. Sure there are degenerate gamblers, coke heads and alcoholics (recovering and otherwise) in the group but when push comes to shove they (sometimes) answer the call.
The books seem to get better as they go on and that is high praise because the first two in the series are the sort of book you cannot put down. Sometimes you start to put them down and think; "ok, ONE more chapter." This third book is also pretty self contained. If you haven't read the first two you will still follow it but there is no reason to not start at the beginning and get to it.