The City's Son

Hidden under the surface of everyday London is a city of monsters and miracles, where wild train spirits stampede over the tracks and glass-skinned dancers with glowing veins light the streets.
  When a devastating betrayal drives her from her home, graffiti artist Beth Bradley stumbles into the secret city, where she find Filius Viae, London's ragged crown prince, just when he needs someone most.  An ancient enemy has returned to the darkness under St Paul's Cathedral, bent on reigniting a centuries-old war, and Beth and Fil find themselves in a desperate race through a bizarre urban wonderland, searching for a way to save the city they both love.

Without a doubt, this has to be the best Young Adult book I have ever read.  And probably one of the prettiest covers I have ever laid my eyes on. 
 
The City's Son has a great, imaginative, story that incorporates all aspects of the city of London and Tom Pollock has created some brilliant, fractured characters to inhabit his London underbelly.  I really liked Pen, Beth's best friend, and her growth throughout the course of the novel, but then I liked all the characters relationships and interactions.  I was also amazed at how every page was firmly rooted in the city, Pollock using London as another character for his intriguing cast of misfits.
  What impressed me the most was that The City's Son reads as an adult novel, and that there were consequences from the actions within the book that can't be erased or corrected by a magical/unbelievable event.  Also the momentum of the plot carried me from being a sceptic into a full fledged fan.

There's an obligatory twist in the tale that I guessed early on, but it doesn't rankle so much as it sets things in motion for the next book, The Glass Republic, which I am eagerly anticipating.  The City's Son is everything I wish Whispers Under Ground would have been, and I would recommend it for any fans of The Rivers of London/Peter Gant series.