An old piece of writing advice is that you should not show all your research. You want it to inform your story without drowning your reader in it.
The same can be true of the fictional background of a story. The writer may know more than she tells the reader about the characters, the past, etc. Think of it as the architecture where the story takes place; it may not get described, but it shapes where the characters can travel.
Ryan's story is intentionally vague on some points, letting the reader fill in the dots. For example, Amanda works for The Firm, and we don't know exactly what that august company does, except that "I knew enough about The Firm to put Stacy and the other partners in a federal penitentiary for a very long time."
Oh yes, Stacy. When the story begins Stacy is firing Amanda. They were rivals, competing for the same chair at The Firm, and Stacy won. But it is not just a business rivalry. It becomes clear that Stacy wants to ruin Amanda's life. Why? Well, that's vague, too.
And then there's Angela (ugh... why name two important characters Angela and Amanda?). She is clearly in the Witness Protection Program for reasons that will eventually become clear.
If this all seems terminally vague, all I can say is, it works. And when Amanda seeks revenge, there is nothing vague about it.
