Others are Opprør (Uprising) (about 1960s type radicalism, 7 books) and Vokterne (The Guardians), an X-files-like science fiction series of 6 for young adults.
They may continue into the large forests of Sweden across the border, or occasionally further, including the Soviet Union, Japan and the United States.
Military and security personnel, industrial workers and working women - often unwed mothers - as well as small farmers and city criminals are among those who populate his books.
His tales mix historical and social realism with the fantastic in the form of ghost stories, local legends and science fiction.
A novel about the Norwegian prewar detective hero Knut Gribb describes the building of a factory that is important later in Fire Søsken.
On her way she passes through a large battle, and she meets the hulder, a supernatural woman that belongs to the subteranians still feared in remote parts of Norway.
The book set a record as the up to then best-selling first novel in a series, and was made into a radio play for the official Norwegian channel NRK.