I don't expect Jean Plaidy books to be full of precise historic analysis or fascinating literary elements, but I do expect them to be solid, reliable, reasonably-accurate, and fun.
This book was mostly historically accurate. I'll give it that at least. But it had several big issues. First of all, the structure of the book was just confusing. About halfway through a chronological tale of Mary Tudor's life, the book pauses, goes back in time 20 years, and spends several chapters following the life of the mother of a dude Mary talks to like five times.
My other big issue is that Mary was just such a boring protagonist. The whole book can be summed up as: "Mary walked in a room, where she was the prettiest woman there, and the most powerful man found her to be witty and charming. Then she went to another palace, where she was also the prettiest, wittiest, funniest girl, beloved by the most powerful man. Etc."
It was just endless, repetitive scenes of how pretty Mary is and how much all powerful men adore her and how she always gets what she wants. And it could be interesting to explore Mary Tudor's seemingly charmed life, if the author had tried to consider all the turmoil and manipulation going on behind the scenes. But according to Plaidy, all of Mary's thoughts were basically: "I'm so pretty and fun and all these men love me."