Terry Pratchett 
Johnny and the Bomb 

It »s May 21 1941, thought Johnny. It »s war.
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 eras – 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 then . . .
The third book in the Johnny Maxwell trilogy.