The book is the story of Catherine of Aragon, starting from her youth and running until roughly her death. It achieves this vast timespan in a relatively small number of pages by consisting mostly of Catherine's voice describing the plot/history and snatches of dialogue, with very few actual scenes as such. So this is obviously bad as it's an uninteresting way to tell a story.

Where accuracy comes in is that one major way Erickson veers from it kills a lot of tension - Henry is a jerk from the moment he and Catherine actually get married. He resents her for taking any attention away from him and he blames her for the deaths of their infants; he's also a big coward and hardly does any work as king. This was all probably done to foreshadow what would later happen, but instead it goes so far as to suck out any suspense. If the only time there was any real love between them was the years between Arthur's death and their marriage, then it's completely unsurprising when he cheats. If he and Wolsey are talking about annulments and stuff after their first son dies, it's completely unsurprising when it actually happens ten years later. Telling a story where every reader knows the ending means that you have to play up how the characters don't know what's going to happen even more. Honestly, there's an entire novel to be made out of the pre-marriage section, by a good writer (although I think P-Gregs has done it as well), ending with the start of what appears to be a happy and loving marriage between C & H.

The other major inaccuracy was likewise done boringly. Erickson gave Catherine an illegitimate half-sister, Maria Juana. Her mother, Aldonza de Ivorra, was real, but the child they had together was a) male and b) born before Fernando married Isabela, where Maria Juana is about Catherine's age. Judging by the Wiki page, Fernando also managed his illegitimate daughters' marriages at home, too, where in the book he sends her to England with Catherine and demands that his daughter arrange a good marriage for her there. This actually could be a pretty cool thing to revolve a story around, but it's handled as boringly as possible. Aldonza is the stereotypical bitchy, entitled mistress à la common depictions of Barbara Castlemaine or Du Barry, and Maria Juana inherits the personality - and we're never really shown why either of them acts this way. This whole thing isn't a strong pillar of the book, Maria Juana goes off and does her own thing and pops up from time to time to be mean. About the time where I left off, she was the mistress of Charles V, who thought he could marry her, and she was telling Cath that if Cath helped that along MJ would testify that Arthur didn't have sex with her. Yes, the Holy Roman Emperor would totally do this. Maybe there was character development after that point, but I doubt it.

Overall, Catherine doesn't do anything and when she does do things, the action is dryly described from a distance.