The British National Archives has over 9000 aviation related documents available for copying. It has been invaluable in my quest for information on my Vampire and its systems. It's not cheap (many documents cost $100 or more to copy),and it takes several weeks for materials to arrive at your doorstep, but the copies are of high quality and generally very complete. If you have a British airplane this is a wonderful site. Check them out at:
Note that the "AP" numbering system has been replaced with the "AIR" system. AIR 10 is where you will want to start. Searching for what you want can be tedious at times, but there is a lot of stuff there.