The title really tells you everything about this book: Catherine de' Medici is the serpent, while Diane de Poitiers is the moon: light, divine, almost chaste - apart from being an adultress, of course!
This is a really good read but of the fun, light, almost novelistic kind, always engaging but undoubtedly `popular' history, so if you're looking for something more academic then look elsewhere. That said, HRH writes really well and brings the decadent C16th French court to vibrant life.
There's quite a long run-up to the Henri/Diane relationship which re-tells the story of Francois I (who, personally, I find far more fascinating that Henri), but then that's inevitable given the impact of Henri's childhood on his psyche, and the age difference between him and Diane.
I have to admit that I've always found the age difference between Henri and Diane quite disturbing: not because she's old enough to be his mother (although she is, of course) but precisely because she actually plays the role of his mother during parts of his disturbed childhood. If the gender roles were reversed and a man who had been a pseudo-father to a girl child then went on to become her lover, would the story still be quite so innocently `romantic'?
In any case, romantic is definitely the key-word for HRH's take on Diane: she's dignified, clever, quiet, suitably loves her old husband in an appropriate manner but never falls `in love' till she does with the adult Henri. And even then, according to HRH's version, she never interferes with his political life and is never interested in power for herself... hmm, not quite the version of Diane de Poitiers' political influence that I've had from other sources.
That said, this is a really good read, and worth switching off your more critical faculties for and treating as a novel in the Philippa Gregory school. So overall a really good read but really rather dubious history.