Anne O’Brien’s novel The Scandalous Duchess may fill a gap.
It starts in 1372 and charts the relationship between Katherine de Swynford and John, Duke of Lancaster, a relationship that broke all the rules but survived the censure of church and state.
Lady Katherine presents herself for a role in London’s Savoy Palace, the wealthy household of John Plantagenet, Duke of Lancaster, and third son of the failing King Edward III. She is an educated, respectable widow with a firm faith and a strong sense of duty to her young family. John, Duke of Lancaster is a Plantagenet, a prince with a reputation for chivalry, is also an ambitious man with England’s destiny in his hands, since his brother’s tragic death and his father’s decline. He also has a new royal-born Castilian wife to help him further those ambitions. Why do they risk all for the sake of love?
Abandoning ‘all moral integrity, all sense of responsibility, all thoughts of God’s grace,’ they embark on an adulterous affair that lasts twenty five years- Katherine destroyed her reputation under a deluge of vicious censure that labelled her she-devil and enchantress. The Duke was attacked by church and state for placing his mistress before the demands of England in a time of war and flaunting her, disgracefully, before his new wife.
Hounded by criticisn, the lovers were ultimately forced to live apart. This wasn’t the end of the affair and ultimately their passion brings them together again ….
An alternative is Alison Weir’s Katherine Swynford: the story of John of Gaunt and his scandalous duchess