Well, that really depends on if you want to focus in on a specific country or manufacturer. For the British, Bruce's "British Aeroplanes 1914-1918" is a solid general reference. For the Germans, a good single reference would be Gray & Thetford's "German Aircraft of the First World War." For the French, try Davilla and Soltan's "French Aircraft of the First World War." Those three alone cover a great number of aircraft types.
Add in a reprint of Jane's All the World's Aircraft 1919, and an Aerosphere 1939 (for all the engines, but tough to find), and that'll keep you reading for a long time.
A lot of data keeps coming to light, so there are those who will always find fault with one reference or another. If you're looking for specific plane information, the Windsock Datafiles on individual planes are really quite nice.