I'm interested in this subject, and I thought of asking here, since there might be other people who's interested aswell. I also like to study the most direct sources I can get to, because of many reasons. Any ideas?
The most direct I can think of is diaries from the pilots, or books written during the war with specs of the airplanes.
EDIT:
To make it a bit more clear, I'm searching to know things such as how they worked in war, and some basic technical specifications.
For a comprehensive view of the technology and deployment of airplanes among the major belligerents of the war, look a John Morrow's "The Great War in the Air." He gets into detail about the technical aspects (i.e. engine production) but I think it's still fairly approachable. It's sort of the book on WWI aviation, so you really can't go wrong there.
For a primary source of an actual WWI pilot: try "Diary of an Unknown Aviator." It's a diary published by an American ace, Elliot White Springs. The diary itself is by a friend of his who was killed flying in the war. It covers the training and combat aspects of it. I think the author flew S.E.5a s. I highly recommend both of these works.
Now, on to specifics: what kind of WWI aviation are you after?
For reconnaissance I can recommend Peter Mead's "The Eye in the Air." A great, albeit old, work on British aerial reconnaissance before, during, and after the war.
For technical specs- how detailed do you want? I've noticed that Wikipedia has solid specs on the major aircraft of the war. Also, check this out. The aerodrome.com. It gives a broad description of a lot of the different airplanes and aces. Sorry if this is too broad, but it would be a nice starting point for technical data if you haven't tried it yet.
Also, a few companies have gone as far as to redesign WWI aircraft for modern use as home built airplanes. They might be a good resource for hard core technical data.
Those are the sources that come to mind off the top of my head, let me know if you have a specific angle in mind and I'll do what I can.
For a fascinating look at the early days of flight from Nevil Shute, who, besides being a great writer, was an aeronautical engineer in England, started his own airship company, served in WWI and WWII, and more - read [Slide Rule] (http://www.amazon.com/Slide-Rule-Nevil-Shute/dp/1842322915/ref=la_B000AQ3XCQ_1_6?s=books&ie=UTF8&qid=1418486170&sr=1-6)