Bradshaw's International Air Guide. Oxford and New York: Old House, 2013 . I'm a sucker for facsimile reproductions like this. Bradshaw's are best known for their compilations of [added: railway] timetables for the Continental traveller, but beginning in 1934 they did the same for air routes. You also get airport information, hotel advertisements, standard air travel rules and regulations (you couldn't take arms on board, for example, unless they were for 'hunting or sporting' and 'packed in such a manner as to cause no danger to persons or things', 159), and a nice pull-out map showing the world's air routes (which clearly reveals the Anglocentric nature of Bradshaw's: it doesn't show any North American or Australian routes because they didn't connect with flights out of Europe).
Robin Holmes. The Battle of Heligoland Bight 1939: The Royal Air Force and the Luftwaffe's Baptism of Fire. London: Grub Street, 2009. The Battle of Heligoland Bight was a daylight raid on Wilhelmshaven on 18 December 1939 in which just over half the RAF force was lost, leading to the suspicion that the bomber might not always get through. This book focuses on the fortunes of one particular Wellington and its crew, but also takes in the wider story of the raid.
Richard Overy. The Bombing War: Europe 1939-1945. London: Allen Lane, 2013. This is a bit of a magnum opus. There are few people as well placed as Overy to write a deep summation of and reflection upon what we know now about strategic bombing in Europe in the Second World War (there's even a chapter on the Soviet experience), and by all accounts he has succeeded brilliantly. I've been waiting for this with some impatience and look forward to reading it (when I have a few weeks to spare -- with endnotes, it's just over 800 pages in length).