Shiers, George, "The First Electron Tube", SCIENTIFIC AMERICAN, March 1969, p. 104.
Tyne, Gerald, SAGA OF THE VACUUM TUBE, PROMPT Publications, 1977 (reprint 1994), pp. 30-83.
Stokes, John, 70 YEARS OF RADIO TUBES AND VALVES, Vestal Press, NY, 1982, pp. 3-9.
Thrower, Keith, HISTORY OF THE BRITISH RADIO VALVE TO 1940, MMA International, 1982, pp 9-13.
How vacuum tubes really work - John Harper's excellent website about thermionic emission and vacuum tube theory--much mathematical detail, at college level.
http://www.radau5.ch/valves.html - A lot of very interesting technical information about vacuum tubes, with PDF files from old books in both English and German, with an outstanding theoretical discussion. The difference between the American and the German techniques is interesting. The American technique usually uses the gain as central parameter in the calculation. The German technique uses the transconductance(durchgriff) as the central parameter, which is a little bit more abstract but since the transconductance is the most constant of all the parameters of the tube, it makes calculations more predictable and more precise.
