Anya Seton wrote this novel in 1954 and it has been republished many times - the version I read was republished in 2006. It is a classic love story based on the long term relationship between Katherine de Roet (Swynford) and John of Gaunt (son of Edwad III and his wife, Philippa). Some records of the relationship, which resulted in their marriage in January 1396, remain but these are few. Seton therefore has used her imagination to write a romantic novel portraying what might have happened by weaving a story of the relationship with events occurring in the fourteenth century such as the Black Death, conflicts with the French and the Scots, riots in London along with political intrigue at Court and within the Church. Once I adjusted to Seton's style of writing I had to finish reading the 574 pages of this novel.
Alison Weir in her biography of Katherine, Katherine Swynford: the story of John of Gaunt and his scandalous duchess includes a section at the end where she discusses the novel and concludes that 'as Anya Seton herself stated - it is based on history, it is a work of fiction'. (p286)
Remembering therefore that Katherine is a work of fiction it is still a good read and possibly has introduced many readers to the world of fourteenth century England.
An article by Tamarra Mazzei To Katherine on her 50th anniversary discusses the impact of Anya Seton's book, Katherine.