Johnny and the Bomb — Terry Pratchett

Johnny Maxwell and his friends have to do something when they find Mrs Tachyon, the local bag lady, semi-conscious in an alley ...as long as it's not the kiss of life.

But there's more to Mrs Tachyon than a squeaky trolley and a bunch of dubious black bags. Somehow she holds the key to different times, different ears -- including the Blackbury Blitz in 1941. Suddenly now isn't the safe place Johnny once thought it was as he finds himself caught up more and more with them...