Katherine Swynford presented Alison Weir with a problem which the author mentioned in her introduction. Although she's the direct ancestor of every British monarch since Henry VII, 6 US Presidents, and Winston Churchill (among other illustrious figures), we know almost nothing about her. Her family was gentry but not notable; her father was a foreign knight and Katherine came to Britain to serve in the household of John of Gaunt's first wife Blanche, Duchess of Lancaster. Katherine married another knight, and after her husband and Blanche died, she became governess to Blanche's two daughters. She also became John of Gaunt's, by then married to Constance of Castille mistress, eventually giving birth to four of his children.
Royal men frequently took mistresses and provided for their children. What they did not do was flaunt their affairs. John's public acknowledgement of Katherine created a scandal; a scandal which increased during the early years of Richard II's reign. Katherine and John parted amicably, and she managed the estates inherited from her husband while John tried to bring Castille under the English Crown. Eventually Constance died and John did the unthinkable - he married his mistress and spent the last few years of his life with her.
Katherine seems to be a very lucky woman, always in the right place at the right time. There's so little information about her because she wasn't remarkable, but her descendants founded a dynasty. It's as if Lizzie Bennet were Queen Victoria's grandmother. Weir doesn't exactly explain how Katherine gained her status, but she demonstrated that Katherine was intelligent and well-liked, as well as strikingly beautiful. Charm, brains, warmth, and a little bit of luck can bring you far in life. In Katherine's case, they brought her into the royal family, and even helped her brother-in-law, a low-born poet and courtier named Geoffrey Chaucer, rise in the royal household.