Gnomon

Near-future Britain is a state in which citizens are constantly observed and democracy has reached a pinnacle of 'transparency.' Every action is seen, every word is recorded and the System has access to thoughts and memories.
When suspected dissident Diana Hunter dies in custody, it marks the first time a citizen has been killed during an interrogation. Mielikki Neith, a trusted state inspector, is assigned to find out what went wrong. Immersing herself in neural recordings of the interrogation, what she finds isn't Hunter but rather a panorama of characters within Hunter's psyche.
Embedded in the memories of these impossible lives lies a code which Neith must decipher to find out what Hunter is hiding. The staggering consequences of what she finds will reverberate throughout the world.