Buddha and the Borderline is a memoir about a woman suffering from BPD -- she cycles through different expressions of the disorder, and for the most part internalizes her suffering and rage in a way that could be characterized as quiet borderline.
My BPD loved one is quiet. Her psychiatrist referred to her as chronically suicidal and she is often so flooded with unexpressed emotion that her version of conversations are confounding. Intense emotions have a way of distorting reality and impacting her cognition. It's like a game of telephone where meaning gets stripped away until the first and last words become something comically different, except it's not funny

She works so hard to mute her feelings that she will disassociate, and other times her face is so blank I'm at a loss as to what is happening in there. I'll find out later, usually by text, that I said or did something to offend or upset her.
She can also be so desperately needy it's unnerving, like a small child has taken over her mind and body, but I have to treat her like an adult or else I'm a monster.
One of the things that helped me make sense of her BPD is knowing a bit of history about how researchers and psychologists viewed BPD then and now. With the DSM, there was a push to create research and diagnostic categories that added clarity for the purposes of better empirically based treatments. This sort of medicalized BPD as a set of criteria (needing 5 out of 9 to be diagnosed per the DSM), although I can see important gains. I can't remember the exact number, but if you were to count the potential combinations of BPD, there are thousands I believe. Plus, it is not uncommon for there to be comorbid diagnoses, making BPD more like a spectrum with varying degrees of severity. I think Gundersen is pushing for an understanding of BPD as a spectrum, similar to autism.
Anyway, if you have a quiet BPD loved one, you may find
James Masterson's In Search of the Real Self to be helpful. It's a bit dated (blames the mother), and doesn't offer skills. It's more about explaining BPD pre-DSM, less as a set of symptoms and more as a way to understand what happens when the development of the self is interrupted.
For skills,
Loving Someone with BPD by Shari Manning is helpful (also
Valerie Porr's Overcoming BPD). Both give many examples of different kinds of BPD issues (not just impulsivity and rage) and the communication and relationship skills needed to prevent us from rescuing and saving to the point of undermining their sense of competence and accountability.
Has your wife been diagnosed with BPD?
In what ways is she a quiet borderline?