I was going through some writings and and analysis of Aristotle's work recently and came across the most fascinating distinction - exoteric and esoteric knowledge.
There's a few different meanings of the words, but exoteric can mean "external / outside / knowledge that's easily ascertained by anyone" and esoteric can mean "internal / secretive / knowledge that's not easily ascertained."
One thing I've started doing in the last year that I've gotten a lot of gain from is trying to find writing by authors and people I respect that were written for a friendly audience.
There's a huge difference in the tone and content of friendly-audience writing (which can be more esoteric) and general-purpose writing (which is frequently exoteric).
Exoteric writing tends to be more sanitized, more generalized, more politically correct, laying out basic premises and assumptions more (instead of assuming skill/background on the part of the reader), and frequently aims to be less offensive.
Things written, said, or spoken for a friendly audience - potentially esoteric - can often cut to the heart of things and provide much more interesting and actionable things to do with it.
Often the greatest advantages come from esoteric writing and teaching - and again, I'll remind you I don't believe in mysticism of any sort, we're just talking about esoteric ways of doing normal daily life activities, or general expansion in arts/commerce/business/governance/law/etc.
So - a recommendation for you - look for esoteric writings. By definition, they're unlikely to be a NYT Bestseller or in the popular categories of Delicious or Amazon.com. But when you find something, it can be incredibly valuable.
Ray Dalio's "Principles" strikes me as a good first example of it. It was originally written internally for his organization, which is already vetted as smart and driven people. Therefore, it contains a lot of rawness and honesty that you wouldn't get in a mass market work which would be more PR-savvy.
Look for things like that. They're hard to find, but potentially worth a lot since they convey rare advantages most people don't have.