Like many others, I'd heard great things about this book, which chronicles the life of Katherine Swynford, mistress and eventually wife to John of Gaunt. While I enjoyed it, that was mostly for the strong plotline and immersive detail, not for the character development or the famous romance.
The book begins a bit slowly, with the oft-repeated plotline of a teenage girl going off to court for the first time, but picks up when Katherine arrives at court and immediately begins to attract the wrong kind of attention. From there it covers several decades of Katherine's life, focusing primarily on her love affair with the Duke of Lancaster, but with several strong subplots dealing with her family and friends and with a few major historical events (mostly peasant uprisings and the political struggles between the nobility and the Commons).
The plot itself is interesting enough to make the 500-page novel a fairly quick read (and in fact it could have been longer; the ending seemed quite rushed). This was an interesting time period and Katherine had an interesting life. The writing itself is good. The characters, however, are rather vaguely drawn, especially the principals. In the best books, the characters come to life so that you feel like you know them as people; that's not the case with Katherine or the Duke, and this is probably why I wasn't particularly moved by their love story, even though there are some well-written love scenes. I was left not entirely sure why they had such an enduring passion for each other, beyond good sex, his wealth and power and her beauty.
Speaking of which, the author describes how beautiful Katherine is more than is strictly necessary; while she doesn't present Katherine as perfect by any means, the portrait is perhaps a bit idealized. And also a bit outdated: there's some evidence in the scant historical record that Katherine wielded power from her position as the Duke's mistress, but here Seton repeatedly emphasizes Katherine's lack of interest in politics, forcibly contrasting her with Alice Perrers, the "wicked" mistress of King Edward III, who (gasp, horror) influences political decisions and gains benefits for herself and her family. Katherine, though, is wonderful and pure because she just follows her heart.... At any rate, I can tell why this book was seen as feminist when it was published in the 1950s, because it focuses on the lives and relationships of women, but from a modern perspective its conception of a woman's appropriate role is quite narrow.
As for the historical detail, it's excellent. The saying that the past is a foreign country is borne out here; Seton has clearly done her research, but not only that, she actually uses it rather than glossing over it. There's a lot of description for those who appreciate that (and I generally do), but Seton also does an excellent job of showing how the characters live and how they're shaped by their society. The sections dealing with the peasant uprisings are especially interesting, although Seton treads a fine line here: she wants the main characters to be "the good guys," which is hard when they're opposing the serfs' obviously righteous demands for freedom, so to a certain extent she tries to avoid the issue by painting the protestors as hotheads and often criminals. But writing both accurately and sympathetically about historical characters who support human bondage is always a tricky business, and although Seton is a little insistent on making her protagonists sympathetic for my personal taste, she strikes an acceptable balance (and to her credit, Katherine eventually does free her serfs).
In the end, I found this to be an enjoyable and somewhat educational book; it is mostly focused on the protagonists' personal lives, so we only see the wider picture when necessary for the plot, but it's still a fascinating portrait of 14th century England. I would recommend it to fans of historical fiction.