WMG / Rivers of London

There is an Anthropomorphic Personification of London and it is Nightingale

Peter often wonders if London as a whole can have a spirit, and what it would be like, but he misses that Nightingale and himself both know the city inside out and practically feel its rhythms. That is the reason Nightingale is getting younger as The Magic Comes Back. At the end of the series Peter will take over that role. He practically lays claim to the position in his "World of Cardboard" Speech to Tyburn in the first book.

Book Six will be a Tyburn centred book

It is provisionally titled "The Hanging Tree", an old name for the Tyburn Gallows, and the other books have all hinted at an upcoming showdown between fan favourite Tyburn and The Folly (as well as with Mama Thames herself. Tyburn also has big plans for kicking over The Masquerade (which, admittedly, is increasingly ropey and thin), so this will be when it all kicks off.
  • Alternately, it'll be another one where the Faceless Man turns up, as he seems to be making an appearance every other book.

The Faceless Man is as old as Nightingale, and also ages backwards.

Broken Homes established that Nightingale's situation isn't unique, so there's no reason to assume the Big Bad can't be subject to the same effect. Rather than "the magician" from the 1960s training up an apprentice who's causing trouble in the 2010s, they're both the same man — another Casterbrook graduate, and a close colleague of Geoffrey Wheatcroft's — who'd dropped out of circulation when he'd gotten too old to keep up the pace as a criminal, only to resume his former crimes now that he's young again. His chimeras are the product of twisted magical techniques he'd looted from Ettersberg, at which he faked being "killed in action" so he'd be free to steal secrets from the Nazi wizards: terrible, powerful secrets he knew the other British wizards would insist on destroying.

This explains why he refers to someone named "Jeffers" during the rooftop confrontation in Moon Over Soho — it's Wheatcroft's old school nickname, which an apprentice of his probably wouldn't use — and why he recognized Alexander Smith from Larry the Lark's "debut", an event that a man agile enough to be jumping off exploding tower blocks likely wouldn't have been old enough to witness if he aged normally.

Lea is the oldest River to appear in the series.

Father and Mother Thames evidently only have children of their own sex and ethnicity, yet Lea is a white woman. Unless handing over the role of river-spirit to a new god/goddess causes the tributaries' spirits to change sex, while retaining their previous racial appearance, Lea can't be a leftover from Father Thames's pre-Great Stink incumbency. The most logical conclusion is that a female Thames-spirit of native British ancestry pre-dated Claudius Tiberius Verica, and Lea is her daughter who has vastly outlived her pre-Roman mum.

Molly can speak, she just refrains from doing so.

Peter's seen that her teeth and tongue aren't human-like, so her vocal cords may also be unusual in their structure. Molly's voice may simply sound strange enough that she avoids using it in order to "pass" as more normal, same as she covers her mouth while eating or snickering.