Arguably the most important paper in the invention of the airplane is a triple paper On Aerial Navigation by Sir George Cayley. The article appeared in three issues of Nicholson's Journal. In this paper, Cayley argues against the ornithopter model and outlines a fixed-wing aircraft that incorporates a a separate system for propulsion and a tail to assist in the control of the airplane. Both ideas were crucial breakthroughs necessary to break out of the ornithopter tradition.
The final proofing has not been done, on these papers, so please report any errors you find.
On Aerial Navigation, Part One.
On Aerial Navigation, Part Two.
On Aerial Navigation, Part Three.