Johnny and the bomb - Terry Pratchett

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.