Well, since I've been held up as an example that doesn't fit the situation, let me throw in as to what I think you should expect.
1. As I read it, there likely is an A/OM. Think about the worst act possible, and learn to detach from it. You don't want to be broadsided by your own emotions if she does reveal an A. If there isn't an A, then bonus!
2. Think about whether an A is a dealbreaker. And think of it in several ways. What if she ends it on her own, and chooses you. Is that ok? What if it ends because you forced her out of it; i.e. it wasn't her choice. Is that ok? What if it hasn't ended and you have to wait it out? Is that ok? You will likely change your opinion on this over time. Start coming to grips with it and what your core values/tolerance/forgiveness angles are.
3. You likely will not learn much from showing her the book. If she isn't in an A, she will claim not to know what it is about. A WAW in an A will lie. She will claim not to know what it is about. We all dream that she will break down, confess, and repent, but it never happens that way.
So you can maybe look at the book and say "Oh my, look what someone wrote in here!" and then watch the reaction. But if she's a WAW in an A, and you say "Do you know what this means?", she's going to take that as a full on accusation and it won't be pretty.
