Managing Psychic Abilities: A Real World Guide for the Highly Sensitive Person

This is the psychic development book that I'd recommend above all else. It assumes ability, but I think if people look at themselves as already possessing it, just needing to figure out what it is and how to control it, it can be valuable in that context. It's also great for people with soft boundaries as some mental exercises on autonomy and assertiveness--or people who are sensitive/empathetic to their own detriment if you want to get metaphysical about it.