This was one of my favourite books as a child. I read and re-read my copy until the covers fell off, and even after reading it so many times I never failed to be thrilled by the story, excited and tense, even knowing the ending.
Chas McGill is a young boy growing up during the bombing raids of WW2. Like all young boys his age he is fascinated by the planes and plays imaginary war games with his gang. One day Chas discovers an extraordinary thing that turns his games into a reality and not only allows him to 'do his bit', but also makes him question what war is actually all about.
Westall wrote another book about Chas, Fathom Five, and although it is good, this for me will always be Westall's standout book. He also made Chas the subject of short stories, 'The Haunting of Chas McGill', which if you enjoy this, and I can't imagine that you won't, you will surely want to read to know what happens to Chas.
Westall also compiled a fascinating book in collaboration with the Imperial War Museum called 'Children of the Blitz'. He wrote The Machine Gunners mostly using his imagination, but when it became popular found that he was receiving letters from people who had grown up in the war who had had real life experiences like Chas'. Intrigued, he did some more research, and this book is a collection of first hand accounts of such episodes, which is fascinating and highly readable.