I’ve seen a lot of people who work with spirits, who only rely on historical information about the spirits they work with to learn about them. I’m not saying that it’s bad to look to historical info for knowledge, but to treat it as absolute truth, denying the UPG of others, is folly.
How do you think the historical information was discovered? Someone wrote down their UPG, and maybe had someone else confirm it.
Just a little bit to think about.
I personally don’t do the research-in-advance thing. I make the connection first and I might look for verification in the documentation afterwards, but I don’t like to go in with expectation, but questions.
Also, many beings I contact are from newer grimoires like Kingdom of Flame, Book of Azazel, and have very little or no historical info on them.
Historical information trumps upg, unless you really, really trust the person that discovered that gnosis or it resonates with you personally
And of course your own upg trumps both
But that historical information is just someone’s UPG from a long time ago. No matter how many people share it.
From a time these beings were far more popular and far more people worked with them.
Is that really true though? In a lot of cultures, the only people who actively worked with these beings were shamans/spiritual leaders of the community.
It appears both you and me have already formed a opinion that isn’t likely to change. I have made mine known and have no interest in arguing it
Very well then, but if you ever feel like reexamining this topic at a later date, just let me know.
Funny, I had this exact same conversation a couple of days ago… I agree that many information is the myths are true, but also many aren’t. Also some are twisted according to the “story teller” and his personal views (or issues).
From that point on though, don’t forget that the spirits, same as everything else around us are expanding, evolving and they’re never stagnant. Their stories didn’t end 3000 years ago. Their stories are still ongoing. By limiting our perception (based on the knowledge we have just from the ancient stories), we only harm ourselves as we become unable to grasp their true nature and potential.
I don’t mean that we have to believe every single UPG we see out there, same as we shouldn’t believe every myth, it’s simply up to the individual and our capability to separate the truth from the bs. 
Hiya! OP is spot on about historical info, mythological depictions, and practitioner records being only a piece of the puzzle.
“Preparatory immersion” (which spans history, mythology, and the magical journals of others) has no other purpose than to link you to the energetic conditions which allowed the discoverer to perceive things in the way that is written down. Even if you don’t believe what you read, the act of research creates an unconscious expectation which resonates with the description; you vibe with the author or account in order to access what they got.
That isn’t reality, though, that’s just piggybacking! 
The people who get into magic to accomplish normal things more easily usually don’t need to go deeper than other people’s work, because if what they access manifests what they want, they either have no impetus to take it farther, or continue going by what worked for them to get what they want.
When you peel back the layers, you find that there’s a lot less individuality and distinction than mythology and history would have you believe. There is no such thing as a ‘line’, much less one to draw- everything available for magical reference is just what the authors could access and understand. Our complete recorded interactions with spiritual forces are really just a pile of boxes we stack together and draw lines between, because of our obsession with categorization and control- there are no lines, much less the lines we use to make the boxes we stack!
‘UPG’ is usually relegated to shamanistic or mystic fields of esoteric interpretation, but that’s just another box. What it really means is to believe in yourself- don’t trust ‘the powers that be’, trust ‘the powers of ME’!
It takes some getting used to, but when you’re used to using yourself, there’s no other feeling like it. You’ll stop treating people and their records as gospels and guides, and starting using them as they were meant to be- the observational equivalent of rumors! Some are fruitful to follow and flesh out your work, but more of them are just ‘loops of interest’, energies that are pretty and flashy and mildly interesting but otherwise useless.
Trust Yourself!
I couldn’t have said that any better myself.