It depends on your interests; no two psychonauts are the same in their priorities, but he has touched on enough topics to have something for most intuitive types. You could do what I did, and pick up a copy of The Portable Jung. It has excerpts from a whole bunch of his writings, so you can either read front to back and have a good general idea of his stuff, or you can pick and choose what is most interesting and then follow the source material to larger volumes of his work on the topic, like the aforementioned Man and His Symbols.
I can tell you where not to start. Aion or the Red Book. Take him to dinner before going in the deep end.
Nothing can stop the nihilistic decline of a civilisation. It is a force of nature and part of the cycle of life and death. There has never been a civilisation that keeps on getting better and better. They all rise and fall.
What I think is important is making the connection with the unconscious parts of the psyche as a means to increase experience in the world. I don't seriously believe we are heading toward a Pagan culture at all. Paganism incorporates a whole functioning society really. But, that doesn't mean you ought not learn about the unconscious and read about the myths. The myths are not literal stories, they resonate on an unconscious level as metaphor and personifications. Understanding the myths and the psyche changes personal experience and improves health. I am 100% certain of that.
Joseph Campbell writes about these things so you could try some of his books if you're interested. He edited the Portable Jung too. But I would read Campbell before you read Jung.
http://www.amazon.com/The-Portable-Jung-Library/dp/0140150706
After you read Jung you can analyse your dreams. That is something I believe is really important, you get direct access to your unconscious and learn things about yourself that increase development and wisdom.
> The idea of ancestry you mentioned is certainly important here too
Well, we are the product of our past ancestors, they are with us always. That is just who we are. Indo-European religion is not Abrahamic, that is an alien religion. So, if you want to really honour who you are you need to learn the myths of your past breathen and reawaken the gods within to feel their spirits.
Portable Carl Jung edited by Joseph Campbell is the one best collection of his writings. My favorite at the moment is his essay on UFO's (Flying Saucers a modern myth). The synchronicity essay is also pretty good.
None of his original works are indispensable. He was a psychologist, not an occultist per se. In Gluckman's college psychology textbook Jung is not even in the index. So not only was he primarily a psychologist, he is now an obsolete psychologist. And his books are not well written. Paragraphs go on for multiple pages to cite only the most obvious example but I can think of others. (I have read a lot of Jung.)
Where he excels is as a scholar. He could dig some really interesting odd stuff out of libraries and he married like the richest woman in Switzerland and never had to worry about anything for his whole life so could just drift intellectually into any odd nook or cranny that he pleased. His books are treasure troves of fascinating trivia. And he is thorough at giving citations so his facts may be checked. Often not easily checked as some of his sources are very obscure, but they can be checked.
This book is a great intro to Jung and the breadth of his work.
Jung's work is so diverse and evolved so much throughout his life that it's good to get a wide perspective. Once you have that you can hone in on the areas of his work that you find interesting.
If you want better, deeper explanations, perhaps check out some books. One of my favorites is The Portable Jung. There are excellent descriptions of functions in it. It greatly helped my understanding of them.
Many people will tell you to read "Man and His Symbols", but only one piece in there is written by Jung himself (everything else is from other Jungian analysts).
If you want a really short book that covers the core ideas in under 150 pages, then "The Undiscovered Self" is probably the best quick introduction.
Joseph Campbell was heavily inspired by Carl Jung. For example JC assembled as an editor various essays CJ wrote over decades into a single anthology called The Portable Jung.
First of all, of course the conversion is one to one. ~~Whoever wrote that article doesn't know what he's talking about.~~ (EDIT: I think the guy does know what he's talking about, but I misunderstood your understanding of it. It's not 1:1 in the sense that it works differently for extroverts and introverts.) There are 16 types, you can change their names, but you can't change the structures of their psyche. The naming is unimportant.
I tried to read the two descriptions you provided and in my opinion they are very vague. However, I thought that the first description was describing an INTP and the second, an INTJ. However, I felt as though these were written from an invidividual's perspective, not through rigorous analysis of the published work. Personal anecdotes are fine, but in the end these are theories and have their basis in a lot of prior study.
I'd recommend a few books:
I haven't found a good book on socionics which goes into type dynamics ... But my most trusted resource is this wiki
An INTP is LII, an INTJ is ILI.
Here's a test that I trust.
The way the J/P preferences come into play is this:
MBTI: In MBTI, the last letter picks your extroverted function. So if you are an INTP, that means that your extroverted function will be your perception function which is intuition for an INTP. Since an INTP is an introvert, their first function will be the introverted judgement function (the function that is left over from the extroversion selection) which is introverted thinking.
Socionics: In socionics, the last letter picks your first function. So INTj (INTP in MBTI) means that your first function is your judgement function. Since an INTj is an introvert, their first function will be their introverted judgment function, introverted thinking.
The difference between them comes from this. Since for extroverts the last letter in MBTI picks the extroverted function and in socionics it picks the first function, the letters remain the same for both. But for introverts, in MBTI the last letter picks the second (the extroverted) function and in socionics it picks the first (the introverted) function, so you have to flip the letter. INTP -> INTj and ENTJ -> ENTj.
Hope this helps.
As for your own personality type, you should first figure out if you're an introvert or an extrovert. It's sort of the easiest one to figure out. (The test should settle it...) But INTPs and INTJs are very different. They are the smartest of the types, but their minds work in completely different ways. Here:
http://www.wikisocion.org/en/index.php?title=Logical_Intuitive_Introtim
http://www.wikisocion.org/en/index.php?title=Intuitive_Logical_Introtim
Also, stay away from forums. Instead read the works of the psychologists who have actually discovered these things. The first website you gave (with the article) was my first resource for socionics, but I later realized that it's not super duper accurate. The wiki is a lot better in that sense.