This is a good addition to the wealth of literature on the Spitfire. The book presents various wartime documents with a solid introduction from historian Dilip Sarkar. It doesn't really feature any new research or previously unseen material, but makes for generally interesting reading.
It is worth noting that although the title of the book is 'The Spitfire Manual 1940', the vast majority of the content is not drawn from the period of the Battle of Britain. Instead the book collates extracts from Johnnie Johnson's log book dated March 1944 to May 1945, a combat report also from 1944, an aircraft movement card from 1941-43, some instructional booklets for fighter pilots, again largely from 1943, and some aircraft recognition profiles of major Axis and Allied aircraft types. This is decent stuff, but doesn't really give a sense of how the Spitfire was being operated in what perhaps was the aircraft's true test, when the Spitfire's excellent performance and combat record really established its reputation among pilots and the British public. As such the book will be of limited use if the Battle of Britain is your area of interest, or if you really want to know what it might have been like to fly Spitfires at that time. The documents are also all facsimile reproductions, and although the print quality is generally good it does decline at various points throughout the book. Neither is it very easy on the eye!
The book has some interesting content, but is not among the best books on the Spitfire (even in this price range), nor is it as good as some of the other historical pocket books that are available.