Mindfulness is Learned from the Inside
Mental health-care professionals and other counselors have a strong tendency to regard Mindfulness as a tool or technique they can adequately understand at a conceptual level, and competently teach after taking a brief course in MBSR, MBCT, ACT etc. This is akin to a PhD in kinesiology, whose only exposure to skiing is an introductory course on a bunny hill, becoming a ski instructor.
People who have longstanding meditation practices and teach Mindfulness, strongly recommend that people considering using any one of a growing number of Mindfulness-based Therapies, should have an ongoing regular meditation practice, sit several longer (at least 5 days) silent meditation retreats, and take at least one high-quality MBSR teacher-training course, ideally from where it all started: http://www.umassmed.edu/cfm/
I read a great deal about meditation-related topics for 30 years before I started a sitting meditation practice. Actually practicing meditation made me realize that the only thing I really learned from all that reading was that I should practice meditation.
Books written by experienced meditators puts into words what we directly experience during meditation. Reading about things one has yet to experience during meditation, feels meaningless & does not really sink in - or - is misunderstood because it's out of context. MBSR teachers will observe this in their students repeatedly - but only teachers who meditate will deeply understand & be able to deal with it skillfully.
An infinite number of other nuanced phenomena need to be first experienced directly & processed by teachers if they're to be of real help to their students.
See Zindel Segal's story of how he realized that in order to teach, he first needed to learn what Mindfulness was about from the inside ie grounded in practice: http://www.mindfulnoggin.com/blog/why-it-took-me-3-years-to-develop-a-mindfulness-practice