Sigrid Undset is best known for her major series on Norwegian life in the Middle Ages -- the three-volume Kristin Lavransdatter and the four-volume The Master of Hestviken. These works earned her the Nobel Prize in Literature in 1928. The medieval novels were actually a relatively late phase in her career; while she had tried her hand at it early on, she couldn't find a publishing house to take that kind of story, so she spent most of her early writing career publishing novels about contemporary life, the breakdown of moral normas, and usually also adulterous affairs in the big city. She returned to the idea of medieval Norway in the aftermath of World War I, having begun to feel that an accurate assessment of the day required doing something that would put more objective distance between us and the world around us, so that we could better see its strengths and weaknesses.
At the same time, she was undergoing a change in her view of the world. Undset's parents had been atheists, although like many European atheists they were churchgoing atheists who regularly attended Lutheran services. She herself was agnostic for much of her early life. But World War I, the difficulty of raising mentally disabled children, and the slow implosion of her marriage led her to doubt that agnosticism was a genuinely viable resting-point for human life. At the age of 42, not long after Kristin Lavransdatter became an international success, she became Catholic. It was a considerable scandal; Catholics were not highly regarded in Lutheran Norway, and being Catholic was regarded as a very anti-Norwegian thing to be. She was attacked for it, and, not being one to lie still, she attacked back, and became derisively known as the "Catholic Lady". She continued her writing, though, and continued to do well. No one ever doubted her ability to write.
World War II would be a disaster for her. She had been vehemently anti-Nazi from the first rise of the regime (she is one of those authors who has as a badge of honor the fact that all her works were banned in Nazi Germany), and had donated her Nobel Prize to raise money for the Finnish army when Stalin invaded Finland. When the Nazis invaded Norway, she fled to the United States. All the work that she had been writing at the outset of the war came to an end; in the US she wrote a number of smaller pieces having to do with the war, but little else, and when she returned to Norway after liberation, writing was not her primary concern, although she did some. She died in 1949 at the age of 67.
The fortnightly book, Undset's Catherine of Siena (and translated by Kate Austin-Lund), is a highly regarded work published posthumously in 1951. Like Catherine, Undset was a Third-Order Dominican, and St. Catherine was one of her favorite saints. Catherine lived in the fourteenth century, not a high point for the Church, and she actively set about the work of reform. Catherine, of course, was named a Doctor of the Church by Pope Paul VI in 1970; one imagines that Undset would have been pleased.
As it happens, this is the third fortnightly book to feature a Catholic saint -- the other two aints being St. Jeanne D'Arc and St. Bernadette Soubirous -- but the first one of the three by a Catholic author (Twain was about as anti-Catholic as a person can get, and Werzel was Jewish).