I haven't read it, but by perusing the book reviews I think that the phrase "highly sensitive person" is basically giving new language to personality traits that are already discussed in psychology using different language. Perhaps phrasing it in different way makes it easier for some to reflect on their own preferences and behaviors minus some of the negative valence around other commonly used descriptors (I mean, I'd definitely prefer to think about/describe myself using the language of "highly sensitive person" rather than prone to anxiety, shy/somewhat avoidant, and/or easily overwhelmed even if they're talking about the same underlying concepts).