https://en.wikipedia.org/wiki/Bible
Pali canon are also a personal favourite. Tao te ching is similarly good.
I havent read the bhagavad gita or the upanishads, but I should do so.
The reality here is that there's a single uncaused God. Even the polytheistic Hindus believe there's only 1 uncaused God(Brahman). But this God is too complex for human understanding. The religions are all bad, very insufficient, explanations of the God with a cultural lens to help in that understanding.
Your importance of your operating manual is to get you to the point that you live at peace with your neighbours and even your enemies. Life is too short to have enemies. Which is then complex about how to avoid ending up with enemies because cultural differences can be the break. Which is the importance of multiple religions.
Critical thinking skills are the most essential tools, so is studying philosophy. You have agency, use it.
Of course you could always abdicate your agency to religion (pick one), or politicians, or influencers, or money/consumerism.
No matter how you dice it, the choice is always yours, and yours only,
Best we can do is heuristics, rules of thumb, etc.: ancestral wisdom based on as much time and experience as possible. Included in those — and among the most useful and reliable — are tips, not for maximizing benefits/successes, but for avoiding almost certain misfortune and ruin.
Hence: avoid harmful substances and behaviors (alcohol, drugs, gambling …); generally avoid/neutralize envy, greed, etc.; avoid killing; teach yourself contentment / how to live with life's basics / how to want what you have instead of trying to get what you want; keep an inner score card instead of a worldly, external one; care for the body; be charitable; beware tampering with complex natural systems (Chesterton's Fence); eschew superfluity; focus on process more than outcomes (what is in your control, not what is outside of it); etc.
Take care of your downside risk, and then acquire free options / expose yourself to positive expected-value uncertainty.
"We win or we learn." Try to win more than you learn over time.
But why is there not a single operating manual? Because the world is composed of vastly different cultures with their own values, norms, and religions and they have different ideas of how the best version of the self is defined.
—Monty Python, 1983 mySelf = Human.new()
defer mySelf.recycle
while myself.not_yet_dead():
mySelf.improve()