My review on Goodreads:
The characters make for an interesting story, and I do not fault the prose in this book, but the character development I found to be deeply flawed.
John of Gaunt is portrayed in one way, while described in another at various times. But more grating is the portrayal of Katherine. The author appears to want us to sympathize with her as a pious and humble woman who is caught up in a love affair she can not control. On the other hand, her actions (those historical ones we can account for), do not bely this character description. As the novel progresses I find myself more and more frustrated with this fictionalized character who comes across as dishonest.
Preferring, as I do, a more historical description of events, I would be inclined to suggest Alison Weir's version of this story instead. If, however, story-telling is preferred by the reader then do consider this Seton book.