Know your enemy

British publication from May1942.  The foreword states, "Some people hold that the ability to say whether a machine is hostile or not is all that is needed.  That may be true.  But to be able to state with certainty the intentions of an approaching aeroplane the observer must know all the types and their normal duties.  The only way to do this is to know at a glance all the more common enemy aeroplanes.  We trust this book will go some way towards supplying that knowledge and thus serving an important national cause."