Some useful titles:
"British Aeroplanes 1914-1918" by J.M. Bruce, Putnam, 1957
"German Aircraft of the First World War" by Peter Gray & Owen Thetford, Putnam, 1962
"Fokker: The Creative Years" by A.R. Weyl, Putnam, 1965 (reprint 1987)
"AIRCO The Aircraft Manufacturing Company" by Mick Davis, Crowood, 2001
"Fokker Aircraft of World War One" by Paul Leaman, Crowood, 2001
"Nieuport Aircraft of World War One" by Ray Sanger, Crowood, 2002
"Aeroplanes of the Royal Aircraft Factory" by Paul R. Hare, Crowood, 1999
"Sopwith Aircraft" by Mick Davis, Crowood, 1999
"Austro-Hungarian Army Aircraft of WWI" and "The German Giants. The German R-Planes 1914-1918", recommended by pometablava,
"French Aircraft of the First World War", recommended by Maveric; I bought them when I was flush with cash. Well worth the money.
Check BookFinder for availability, even "British Aeroplanes 1914-1918" is still on offer. At a price.