At Rice University is what has to be the premier early aviation collection anywhere. The finding aid is here:
http://www.rice.edu/fondren/woodson/mss/ms069.html
They have stuff I have never even heard of, including a signed portrait of Bleriot, drawings, schematics, photos articles, you name it. Tons and tons of stuff most of it from before world war 2, boxes and boxes from WWI, and even more from the pioneer era.
For a $25 research fee plus the cost of copying, they will send you copies.
I just got a 50 page report by Glenn Martin on the development of airplane manufacturing given to the Royal Aeronautical Society in 1931. I also found a folder titled Mexican Aviation 1913-1914.
An astonishing collection.