I think "London Rules" is the best Slough House book so far. It brings together the same elements used in the earlier books but each element has grown stronger, is used with greater assurance and has been combined with its peers perfectly to make the ultimate Slough House book.
"London Rules" has a violent prologue that reminded me of one of those young woman / old woman optical illusion drawings. I saw the scene perfectly in my head, tragic but familiar, up until the last paragraph, when everything changed and yet everything remained the same. This way of leading me to see the familiar differently and surprise me while he does it, it what makes Mick Herron's Slough House books so appealing.
After the prologue, the book returns to the usual pattern of starting and ending with an almost whimsically lyrical description of Slough House. This time it is not the wind that is visiting Slough House but personifications of Dawn and Day and Dusk. These pieces are well enough written to be memorable in their own right but they are more closely integrated into the story's content and tone than in earlier books so that what might seem sardonically decorative becomes a kind of Greek Chorus, obliquely guiding the reader.
One of the things I enjoy about the Slough House books is how fearlessly, sometimes even viciously, they comment on the current British political culture. The most brutal and most nuanced assaults are made by Jackson Lamb and so might be seen as part of his irascible persona ("There's a Donal Trump Junior?", Lamb said, "And just when I thought things couldn't get any worse.") but the disdain for the people who made insanity of Brexit and Trump possible is shared by most of the characters in the book except for the shamelessly self-serving Pols themselves.
This contemporary pulse-taking is also more than decorative. It provides the issues that drive the plot, giving the plot more credibility and showing us the damage that these people of "middling ability but supreme self-confidence" are doing to us.
The plot is clever and is rolled out with such skill that events continue both to make sense and to surprise. The tension is high right up to the final page. There is intrigue and violence and betrayal and that's just between people on the same side. The terrorist threat here is sadly credible and disturbingly plausible.
I've seen American booksellers refer to the Slough House books as the "Jackson Lamb Series". This labelling demonstrates the same unwillingness to embrace what English books are really about that led to US publishers changing "Rivers of London" to "Midnight Riot" and "The Philosopher's Stone" becoming "The Sorcerer's Stone", because the crowning glory of the Slough House books are the characters that populate them.
Jackson Lamb's gravity bends the orbit of the people around him and sets the rhythm of their lives but these books are not really about him. They are about the idea behind Slough House, a purgatory for spook screw-ups, the people that would stay in such a place and the culture that would find such a place necessary.
The result is an ensemble cast inside both Slough House and Regent's Park (where the shiny, haven't-screwed-up-yet spies live) that gets stronger with every book.
In "London Rules" we do see more deeply into Jackson Lamb but we spend most of our time looking through the eyes of broken spies, whether they live in Slough House or not, and see how they live with the war between their weaknesses and their hopes. We learn a lot about desperation and self-delusion, leavened occasionally with a little hope. I particularly enjoyed seeing Roddie Ho so deeply engaged in self-deception that he becomes impervious to interrogation techniques designed to play on his fear and doubt.
"London Rules" is an excellent spy novel and a good action-packed thriller but it is also a mirror to our current times and an invitation to recognise that self-delusion, confidence without ability and the pursuit of personal power at the expense of personal integrity are a plague on our society.