This resource assembles some links to works that are cited in Exploring GIS, or that provide additional information. I don't think that a list of links is much use, so there is a short explanation for each.
Wm. Randolph Franklin has a lot of material about viewsheds (and material about his earlier work on TINs, including the historical code from the 1970s)
The work by David Douglas (cited as a University of Ottawa Working Paper in the First Edition) appeared in Cartographica with a 1994 date (a journal not always exactly in synch with the calendar...). The article Least cost path in GIS using an accumulated cost surface and slope lines was published in Cartographica Vol. 31, No. 3, Autumn 1994 pp. 37-51.
David has an updated version of this research posted at his current location in Sweden.