I am interested in the MBTI, and have read a few of Keirsey's book (Please Understand Me II, twice), Gifts Differing (Meyers), Conscious Orientation (van der Hoop), The Psychology of C G Jung (Jacobi), and Jung's Collected Work Book 6 Personality Types.

I may not be right, and i have a definite opinion, but i can try to share what i know.

For starters, know that Keirsey's four types, and Jung's eight types are completely different things that just happen to coincide (van der Hoop mentioned it, Keirsey did the amazing work on it). Keirsey went further and explained the actual functions too, but there he is completely wrong. Keirsey is probably a shy extravert and, not understanding that, misinformed millions as to what the I/E scale is. Whereas Jung has it as the most important, Keirsey hardly recognizes it.

For the record, i am INTJ. Currently developing F (Jacobi's lifecycle).