We don't know when Katherine Swynford was born, how many siblings she had, what she looked like, what she wrote or spoke like, what her seal looked like, or why she died. In fact, she is a complete cypher to the 21st century. Weir does the best she can to piece together what few documents and sketches of long-gone monuments that are left to give us clues, but there is very little to work with. Katherine was the mistress, and then third wife, of John of Gaunt (son of King Edward III, uncle to King Richard II, and father of King Henry IV). Her illegitimate (although later legitimized) children by John, the Beauforts, were the ancestors of rulers of Scotland, England, and Aragon.
This book helped me understand fourteenth century European politics and the eventual Wars of the Roses, but unfortunately, I still know as little about Katherine as I did at the start.