For a self help book, this is pretty average. I appreciate the focus on such a misunderstood trait, but neither author has any credentials as an expert; they are both just self-identified sensitive people. If there were more citations and less "psychologists say...", I'd be more inclined to accept the advice. A lot of the book reads like a blog post. They promote their own website multiple times, there's "testimonials" from users, too much text is length-filler. It bothered me that they highlighted examples of historical figures' accomplishments to inspire sensitivity, yet they acknowledge that they cannot be sure the person actually was sensitive. Seeing such high reviews of this book, I expected a lot more. 

On the positive side, if you're looking for some inspiring words instead of actual research, maybe this would help. While the advice is superficial it could help someone see themselves differently if they experience the trait.