The Hanging Tree by Ben Aaronovitch

Peter Grant, one of two official wizards of the London police force, owes his life to Lady Tyburn, one of the genius loci of the city’s numerous rivers. So when she wakes him with a pre-dawn phone call to say that one of her daughter’s friends has had an accident — the “fatal” is unstated but implied — and that she wants him to keep her daughter from being implicated in the investigation, he answers as he promised. “Yes ma’am, no ma’am, three bags full ma’am.” She adds that Nightingale, his supervisor in both policing and wizarding, is not to know. Peter replies that Tyburn has made herself crystal clear. And then calls Nightingale immediately.

“I rather think I’d have to have taken an interest in any case,” said Nightingale once I’d briefed him. “Still, I shall endeavour to adopt a façade of ignorance until such time as you need me.” He paused and then said: “And you will let me know when that moment arrives.” It was not a question.
“Yes sir,” I said, and hung up wondering why everyone felt the need to be so emphatic at this time of the morning. (p. 2)

Given Peter’s task of keeping someone unimplicated in a police investigation, he can’t exactly go digging through computerized records to find out what is already in the system about the incident, so he calls Detective Constable Guleed, who he knows is doing night shift for Homicide Assessment that week.

“Hi Peter,” she said. Behind her I could hear a hushed indoor ambience and people being professional.
I asked whether she’d heard of a shout in Knightsbridge, a suspicious drug-related death.
“Why do you want to know?” asked Guleed, which I suspected meant she was on the scene.
In the background I heard a vast and familiar Mancunian voice demanding to know who Guleed was talking to — DCI Alexander Seawoll. Who, as SIO, shouldn’t even be out of bed until the Homicide Assessment Team had finished their work.
“It’s Peter,” she called back. “He wants to know about our suspicious death.”
“Tell him if it’s not one of his he can fuck off,” said Seawoll.
“Do you have an interest in this?” asked Guleed.
“There may be some related issues,” I said, which was sort of true given that Tyburn’s daughter was involved. I heard Guleen pass this on and some grumbled swearing from Seawoll.
“Tell him to get his arse down here pronto,” he said. (pp. 3–4)

Seawoll was first introduced as someone who, like many Northerners with issues, had moved to London as a cheap alternative to psychotherapy. (Back when moving to London could be anything like cheap, presumably.) He hates Peter’s section and the “weird bollocks” and “X-Files shit” that it drags into proper policing, but will eventually grudgingly admit that they are both on the same side. That admission, however, will definitely not extend to doing any favors for Tyburn or her daughter. Peter is well and truly between the Northman and the deep London river. And that’s before Tyburn’s daughter Olivia admits that she bought the drugs that led to the suspicious death.

The case gets more complicated from there because a group of wealthy London teens and young adults partying in an apartment owned by an offshore trust and located in a building with diplomatic levels of security are bound to have complex connections and plenty of pull. Police work is never straightforward at that level, as Peter’s commitment to Tyburn shows. To make matters worse, two people from previous novels choose this time to cause problems for the Folly, as Peter and Nightingale are collectively known. The Faceless Man, the Folly’s most dangerous opponent to date is taunting them, hinting at greater action. And one Reynard, who may or not be fae but definitely has connections to that side, says that he can get Sir Isaac Newton’s lost Third Treatise on alchemy, magic, and more.

As Peter pursues leads on both the drug overdose — which closer inspection reveals also had a magical aspect — and the lost treatise, he unhappily discovers that the Folly are far from the only ones who would like to get their hands on the book. His American counterparts have sent over a crack team, as Peter learns from an FBI agent he befriended in Whispers Under Ground. The special relationship between the US and UK governments is specially unhappy at the level of their respective magical departments, and the Americans’ presence is not good news for Peter. When the Rivers of London series started, Peter was led to understand that the magical tradition he became part of was the only one in Britain. He has since learned that the respectable British men of whom Nightingale is the last were as blinkered about magic as they were about so many other parts of life, and there were other practices that they didn’t have the faintest notion about. Peter discovers another in the person of Viscountess Helena Linden-Limmer, who not only learned magic through a maternal line but also gleaned practices from time spent in India. She, too, has an interest in the Third Treatise.

Aaronovitch manages all of the complications with aplomb, and the pace of incidents never slows. Peter and Nightingale are juggling numerous fragile and volatile balls, and if that’s not enough, one particular bureaucrat is coming after them as well, trying to tie the Folly down and, not incidentally, get some personal revenge on Peter for an earlier incident at Covent Garden. Fortunately, Peter has been learning to speak bureaucrat. When presented with a pointed question designed for him to admit the Folly couldn’t cover its tasks, he replies

“If I could speak to that, sir,” I said. “The Special Assessment Unit has recently instituted a programme of capacity expansion in order to build general operational robustness and provide a more efficient service to our partner OCUs when dealing with both Falcon [magical] and pseudo-Falcon incidents. The first phase of which is already underway.”
I noticed that Stephanopoulous [another non-bureaucrat police officer] was hiding her mouth with her hand.
Folsom [the bureaucrat], who should have known better, took the bait.
“The first phase being?” he asked
“Strengthening our specialist support, particularly in the forensic and medical area, with a view to providing a continuous on-call service to investigation teams that might need them, coupled with the development of a best-practice guide for use in dealing with suspected Falcon related incidents and investigations,” I said, and heard Seawoll smother a cough — at least I assume it was a cough. (p. 210)

The Hanging Tree can get tense, but the trademark humor is never far behind. Peter’s bafflegab shows how Aaronovitch is funny at the sentence and paragraph level without losing sight of moving the story along while also placing interactions for future stories to build on. Eventually the strands of the investigations start to come together. The main question is, have the Folly figured things out too late? The explosive finish settles some accounts, and leaves others newly opened. Fortunately, the series continues. Time for me to go find a copy of Lies Sleeping.