His first book was Sagaen om de norske Northrop-flyene på Island, published in 1981, about the No. 330 Squadron RNoAF while it was stationed in Iceland during the Second World War. He followed up with Fornebu 9. april in 1990, about the German invasion of Oslo Airport, Fornebu on 9 April 1940. For this and later books he established his own publishing company, Wings Forlag. Guhnfeldt's third book was Bomb Gestapo-hovedkvarteret! about the Oslo Mosquito raid on 25 September 1942. The latter two were compiled based on among other things interviews with four hundred people in seven countries. For the latter two works Guhnfeldt was awarded Fritt Ord Honor Award in 1996 for his "unique, historical authorship built upon meticulous outreaching research".
Later books include a biography of Marius Eriksen (2002), Tarald Weisteen (2004) and Gerd Vold Hurum (2006). He then wrote a series about the Supermarine Spitfire, Spittfire Saga. Two of the volumes have won the Norwegian Aviation Book of the Year, in 2010 and 2011.
Sagaen om de norske Northrop-flyene på Island : – og ett som ble berget for ettertiden, 1981, Sem og Stenersen. ISBN82-7046-025-7