I've only just started reading it, but already he's saying some interesting things about what historical fiction is, and what history is. A few years ago, there was a bit of a stoush about Kate Grenville's book, The Secret River, with historians complaining that she'd taken liberties with "the facts". Thom makes some good points about this, such as:
To be blunt about it, much of the history of many countries and states is based on delusion, propaganda, misinformation, and omission.
Certainly, I think people now realise that, for example, in war that the history is written usually by the victors. And that the history of women and the poor is almost non-existent because scholars and historians of the time believed it wasn't relevant or important. Thom also says:
A good historical novelist has the same obligations as a good historian: to convey a truthful history, not perpetuate pretty myths.
He seems to be the kind of historical novelist who prides himself on deep research and accuracy, and perhaps that's a choice you make when you write historical fiction - whether you are going to stick to the facts you discover, or make history fit your story/plot.
I'm sure you've heard the saying, "Never let the truth stand in the way of a good story". With some historical fiction, it can be more a case of truth hampering a well-constructed plot. There's nothing worse than a plot that ambles along in a series of small episodes that don't ultimately go anywhere, and sometimes history is like that. Life does just trot along. Perhaps the key is in choosing a period in history where there is some cataclysmic event that you can lead up to, that will be your natural climax. From there, you have to make sure your characters also have the same rising arc in their personal journey through the story.
In revising Pirate X, I used, as extra research, two new books that hadn't been published when I wrote earlier drafts. I decided to take the new (different) material into account and change my story, but ultimately what I really focused on was the characters and how events affected and changed them. I think there is a spectrum in this genre - at the one end you have HISTORICAL fiction, where the author sticks absolutely to the facts and can cite a bibliography.
At the other end is historical FICTION, where history is a background but the author adapts if necessary. Pirate X is much nearer the first than the second, but I have quite a few imaginary characters in among the "real" ones. The key for me is to try to imagine what it was like back then, to imagine myself into my characters, and see through their eyes. That's the real challenge for a novelist.