This book is useful whether you would consider yourself a "disappearing woman" or a "Woman of Substance" (both terms the author uses frequently throughout this book), however it is not without faults and many of the author's assertions about what women are like, in general, seems too rooted in her own personal life.
Loving Him without Losing You useful falls in to the trap of advising a woman to change herself to become more suitable to men, which feels a lot like the same old crap we women have always been dealt. It advises against being "too clingy," "too emotional," and variations on these themes, because if you happen to possess these qualities, then you will always drive men away and never learn to become a "Woman of Substance." The author also asserts that "most" women are "disappearing women" and that many, many women likely also have Borderline Personality Disorder to one degree or another; I don't believe either of these statements and the author is unable to provide any hard data to back them up.
The information in this book that was useful to me were the bits about identifying emotionally abusive behaviors (especially subtle ones, like gaslighting, belittling comments, and stonewalling), recognizing my partner's passive-aggressive behavior, learning to trust my instincts (again), and using those tools to target and put a stop to the cycle of passive-aggression and emotional abuse in which I had found myself.
Several of the exercises suggested by the author were useful in identifying how and how much I had been affected by the gradually increasing emotional abuse in my marriage and for helping me feel like the choices I'd already made to support my wellbeing (spending more time apart, investing more in myself, demanding that my husband seek counseling or move out, attending church again, making more time for fitness and solitude, etc) were, in fact, the right things to do for myself and my marriage.
Other exercises were less useful for me, in particular those relating to the Shadow Self (I am already well acquainted and happy to know her), but would certainly be useful to others, both men and women, and especially useful to anyone who struggles with feelings of shame or with projecting their negative qualities on to others.
The author has included an extensive recommended reading list in the back of the book, from which I've already chosen several books to add to my queue.
Loving Him without Losing You would be most useful for people (both men and women) who are truly suffering from the symptoms of BPD and the devastating impact it has on romantic partners, but even though I do not suffer from BPD and therefore will not follow the author's advice to change my personal level of emotionality or otherwise tone myself down in order to make a man more comfortable, I found much of the book's advice useful, I found within the book the emotional affirmation I needed to get back in touch with all of my personal strengths again, and I got a lot out of the exercises included within. Reading it has helped me feel confident about the steps I've taken to regain the balance in my life and my satisfaction with it.