I first learned about the Myers-Briggs theory of personality type (sixteen of them) in about 1993, and found it fascinating. One of the books I read on the topic was 'Please Understand Me' by Davie Keirsey and Marilyn Bates, which proposed that there are actually four broad strands of personality, which they call 'temperament'. I thought the book interesting, but a bit stereotyped (even though the section on children and temperament was extremely good, helping us pinpoint the temperament of one of our sons instantly - that section of the book could have been written with him in mind!)
Years later, I joined a discussion group for temperament theory, and eventually read David Keirsey's second book, Please Understand Me II'. It was a lot more helpful. As I learned more, however, I felt there had to be a closer correlation with Myers-Briggs theory than Keirsey wanted to admit - and that's exactly what Linda Berens believes.
So I bought some of her books - unfortunately, only available in the USA, but with the US dollar being so weak, the postage to Europe isn't too expensive from Amazon or other American sites. 'Introduction to Temperament' has a really well-written introduction to Keirseyan temperament theory, with tables, cartoon-style graphics and clear explanations, which make it much simpler to grasp than Keirsey's book - although, inevitably, there isn't as much depth since it's a relatively short booklet.
All in all, a superb overview, and highly recommended for anyone wanting to know more about this topic, although it would be good to read Keirsey's book afterwards, to gain more depth of understanding.