If you're looking for books, please post in Book Discussion and Reviews with a Seeking Recommendation tag but in the case of a query as vague as yours… ok, I'll try my best: the universalist approach (same as
) has long gone out of fashion now that so much information is available even for the smallest occult system. In fact, I'd be very suspicious of any author claiming that he or she had found more than the most superficial common denominator between various traditions. Right now I'm reading
How to Measure and Deepen Your Spiritual Realization by William Bodri and Nan Huai-Chin - I'm only a few couple of dozen pages into the book but I wonder how much more I can stand, and it's only Asian traditions they're mangling to conform to their own spurious metric; I don't even want to know what would have happened if those guys had included Middle Eastern, European, and indigenous ones, although they made a stab at it in the introduction ("As Mohammed, Plato, Buddha, Lao Tse and Confucius well knew…", horrible).
In my opinion, different traditions and systems don't contain different pieces of the puzzle because
there is no puzzle, period. They all have wildly different premises, originated in different places and times, were a response to different spiritual problems other cultures couldn't have cared less about, etc. What I do enjoy though are books that attempt to compare ancient philosophies with modern ones, such as in
Contexts and Dialogue: Yogācāra Buddhism and Modern Psychology on the Subliminal Mind by Tao Jiang where the author juxtaposes Freud's and Jung's ideas about the Unconscious (already huge differences between them) and the Mahayana Buddhist concept of storage consciousness (
ālaya-vijñāna). It's a serious scholarly work, so the author doesn't have an universalist ax to grind and very honestly highlights their considerable differences. Had he overemphasised the commonalities between the three systems, his colleagues from the academy would have probably torn him to pieces, and rightly so.
You can trace the development of various traditions and occult systems along timelines, or try to spot how they influenced one another but any all-inclusive science, rationale, wisdom, or explanation? No such luck, I'm afraid.
