Complex sentence: This work formed the basis of Undset receiving the 1928 Nobel Prize in Literature, "principally for her powerful descriptions of Northern life during the Middle Ages"; her work is much admired for its historical and ethnological accuracy.
Split into simpler sentences: This work formed the basis of Undset receiving the 1928 Nobel Prize in Literature, which was awarded to her "principally for her powerful descriptions of Northern life during the Middle Ages". Her work is much admired for its historical and ethnological accuracy.