The History of Fredrikstad by Harald Hals is the standard local history, thorough on the fortification period and the Glomma river trade. In Norwegian only, but worth tracking down for those who read the language.
Norwegian Fortresses by Nils Petter Thuesen surveys the major defensive works along the Swedish border, with a substantial chapter on Fredrikstad and Kongsten. Good context for understanding what you're looking at.
Hunger by Knut Hamsun — not about Fredrikstad specifically, but set along the Oslofjord coastal communities in the late 19th century and gives a powerful picture of the region's economic conditions at the time the town was transitioning from military to industrial importance.