Here are a few references. The items in bold are very good for information and good research:
Cross & Cockade Int. (GB) Vol. 3 #3, 1973
Cross & Cockade Int. (GB) Vol. 6 #3 Fold out Cutaway diagram. 1975.
Cross & Cockade Int. (GB) Vol. 34 #4 Pp.1- ? 2003.
Cross & Cockade USA Vol. 1 #1 p.36 1960
Cross & Cockade USA Vol. 5 #1 Pp.1-29, 1964
Cross & Cockade USA Vol. 18 #2 Pp.164-176, 1977
Cross & Cockade USA. Vol. 21 #1 Pp. 81-90. Jasta 14, 1980.
Cross & Cockade USA Vol. 23 #4 Pp.318-334 Baümer & Dr.1 204/17, 1982.
Fighting Triplanes by E. Hadingham, Macmillian Pub. 1968.
Fokker Dr.I, by J.M.Bruce, Profile Pub. #55, 1965
Fokker Dr.I Aces of WW1, by Van Wyngarden/Franks, Osprey Pub. Aircraft of the Aces series #40, 2001. Also there is a Datafile Special on the type and machines assigned to units by this research fiend and historian.
Fokker Dr.I …A Reappraisal by A.Ferko & P.Grosz, Air Enthusiast Eight. Pp. 9-26,
Fokker Dr.I Datafile # 5 by R.Rimell, Albatros Pub. Ltd. 1987.
Fokker Dr.I Drawings by Dan San Abbott, WW1 Aero #122, 1988
Fokker Fighters by A. Imrie, Vintage Warbird Series #6, Arms & Armour Press.1989.
Fokker Dr.I, Flugzeuge die Geschichte Machten by J. Kranzhoff, Motorbuch Verlag, 1994. Fokker Dr.I Special Datafile by R.Rimell, Albatros Pub. Ltd. 1991.
Fokker Dr.I Triplane by Paul Leaman, Ian Allen Pub. 2003.
Fokker Triplane-ology by A. Imrie, C&C Int. Vol.23 #4, Pp.57-64, 1995.
Fokker Triplanes in Service by Dan San Abbott, Over The Front Vol 5.#4 Pp.326-339. 1990.
Fokker Triplane by A. Imrie. Arms & Armour Press. 1992.
German Fighter Units - June 1917-1918 by A. Imrie, Osprey Pub. 1978.
Pictorial History of the German Army Air Service by A.Imrie, Ian Allen Pub., 1971.