City of Shadows — Celia Rees

In the midsummer of his twelfth birthday, Davey, his twin cousins and his sister Kate, embark on a tour of the infamous underground city in Davey's hometown. As the tour progresses, Davey finds himself separated from the others, and inside an eerie chamber. Here he is drawn to a large and hazy-glassed mirror. In a moment, Davey's reflection blurs, and he finds himself back on the streets of the ancient city - but the residents are not of his time - they are ghosts, and some do not take kindly to those from 'the other side'. Davey encounters some friendly spirits, too - but they urge him to try to make it back to his own time, before an evil ghost crew discovers he is mortal. A frantic search for his own world begins, and Davey makes it just in time. But he senses there is unfinished business and, come Hallowe'en, the ghosts will be on the lookout for him - and this time they will be visiting his world . . .