Crown of acorns - Catherine Fisher

In an absorbing mystery thriller, a teenage girl with a past arrives in a city: new name, new identity, new foster family. She has chosen the city herself, and is fascinated by its harmony and beauty, but is clearly in fear of discovery. She is nursing a secret from her early childhood, a secret that produces new terrors for her the moment she fears her identity has been spotted.
A parallel narrative tells of a young architect's apprentice, Zak, in 1750 - working with Jonathan Forrest, a man obsessed with past Druidic mysteries and a new architectural vision for the city. He plans to create the world's first circular terraced street, the King's Circus - a plan greeted with scorn and derision. Zac soon realises there's more than just obsession with an architectural vision; there is some secret associated with building a hidden chamber in the centre of the Circus. But Zac himself has his own confused and highly destructive agenda ...
These narratives are framed by the voice of Bladud - mythical first builder of the city, destined to die in trying to fly. And ultimately his narrative brings all together in a clever and brilliantly intriguing climax.