Historical, very fictional novel about Katherine Swynford, mistress and later wife of John of Gaunt, by Anya Seton. It has an article in Wikipedia claiming it as one of the first historical novels--I guess it does try more for verisimilitude than Jane Porter's Scottish Chiefs--and maybe it is. It certainly has that genre's faults, despite being published in 1954: comma splices, unnecessary fragments, even a "he might of had" error. But it was medieval geekery, and helped me straighten out Edward III's children. Katherine de Roet Swynford Lancaster-Gaunt's son John's son John's daughter Margaret's son was Henry VII, who married her daughter Joan Beaufort's daughter Cecily Neville's son Edward IV's daughter Elizabeth of York. I still don't care about the Stuarts or Hanoverian kings, but the Plantagenets from Henry II through both branches to the Tudors are my thing.