The teacher is essential in almost any type of education, not to mention an area as important as family planning.  No book can adequately teach a human skill.  You can't ask a book questions.  The printed page cannot sympathize with your needs and concerns, or work through them with you.  Even materials specially prepared for NFP education cannot give the guidance of a well-trained practitioner.  You may think you are saving money in bypassing personal instruction, but what you want is a reliable natural method.  It can only be learned properly with professional guidance.