I think this is the strongest novel Charles Cumming has produced to date. He has matured quite a bit since his first novel, A Spy By Nature. It was a good decision to re-visit the main character from the first novel, Alec Milius, who is now in exile in Madrid, working for a British finance company. He fills his time having an affair with his boss's wife, and being paranoid that enemies from his previous life in Britain working for MI5 are still out to get him. His counter-surveillance measures he carries out partly out of necessity, but also partly because he enjoys the secret life, and hasn't been able to let go of it.
The setting in Madrid is good, a refreshing break from the previous two novels set in London. Alec Milius is asked to explore the possibility of his finance company investing in Basque country, and in the course of various meetings, he spends an evening with a former Basque activist and politician. They get on, and agree to meet up at a later date. But on his way to meet Alec, the politician disappears. Suspecting foul play, Alec cannot help himself but investigate, and in doing so, he gets drawn into Basque politics, ETA, and back into the murky world of espionage.
It's not a perfect thriller, but it's an improvement on his first two novels. The plotting is smoother and more effective. Unlike other reviewers, I would argue that ending is not that good - for me it was a contrivance too far. I couldn't fault anything up to that point. This is mostly because Alex Milius is such a convincing, fascinating creation. Charles Cumming makes a passing reference in the novel to The Talented Mr Ripley by Patricia Highsmith, which is a fair comparison. Both Ripley and Milius have tremendous talents, but also the seeds of their own destruction within their character flaws. The espionage theme suggests more Le Carre than Highsmith, and again, I don't think it's an unfair comparison to Le Carre. Good espionage writers understand that the interest is not as much in the plot as in the characters - the kind of people who can carry out ethically dubious activities for a perceived higher cause are by their nature, flawed. This is what fascinates the reader, not which country is plotting against who, how and with what fancy gadgetry.
I can only hope that Alec Milius returns in a new novel, but I think that Charles Cumming was right to let him have a break between novels. Aside from the ending, this is a near perfect espionage thriller.