Tale of Time City - Jones, Diana Wynne

When Vivian is evacuated from London in 1939, she expects to be staying in the countryside. Instead, she is whisked away to Time City – a place that exists outside time and space.It is a strange and remarkable place, where technology rules – yet important events of both past and future are marked by the appearance of mysterious Time Ghosts.Here, a Time Patrol works to preserve historical events – but unknown rogue time-travellers are plotting to take control and are stealing the wards that protect the city. If they succeed, Time City and History as we know it will both be destroyed.Jonathan and Sam are convinced that Vivian can help to save their home – for, astonishingly, she appears as a Time Ghost herself in a forgotten part of the city. But how can she possibly know what to do, when the important event hasn’t even happened yet?!