I agree with several reviewers that the book is repetitious but I never found it tedious. I also agree that Catherine de'Medici gets lost in the telling. The book might have best been posed as a love story between Henri II and Diane de Poitiers or a sociological look at Renaissance life.
The book seems to be written in discreet chapters with little concern for the overall narrative structure, although the book does progress sporadically from the rule of Francois I to Charles IX.
Now, you might ask, why have I given it four stars? The answer is simple: I liked the book for its digression into the minutia of the daily life of the Renaissance Courts of Francois I and Henri II.
God is in the details (Le bon Dieu est dans le detail-Flaubert)and reading Princess Michael of Kent's imagining of the French court is to be dazzled by the details.
No fact is too trivial for her to catalog and discuss. For instance, She delves into the social and sexual practices of the nobles with a eye for the mundane and quotidian. She discusses the utensils they use at dinner and the clothes they wear or don't wear--such as undergarments.
She also looks closely at the the familial relations and the political machinations that arise from those relationships and she discusses the wars between the Renaissance Kings and their petty and brutal bids for power. There are any even side-roads into English politics and appearances of Henry VIII, Anne Boleyn, Mary Tudor, and Mary, Queen of Scots.
And within this broad historical panorama, we learn about dog breeding, hunting, best sexual positions for conception, use of cosmetics, hygiene (or lack thereof) of the royals, de Medici's use of alchemy, soothsayers, astrology, and poison, expansion of Paris, Renaissance gardening and architecture.
In the end, as I read the work, I felt I had an understanding of one aspect of French society--the court. Perhaps, a criticism is that we don't see the filth and poverty of the peasants but, of course, that was never her aim. All-in-all I found the book to be a pleasant, breezy, romp through a complicated and brutal period of French History. And in that description lies both the weakness and strength of the book.