Bible

Start with the originals--Torah, Bible, Koran, Vedas, Buddhist Canons, Greco-Roman Mythologies, Avesta, Enûma Eliš--then work your way out toward commentaries, sects, cults, precursors, apocrypha, heresies, schisms, etc. The problem with any condensed "Grand Tour of Religion" is the inevitable editorial bias (i.e. adopting someone else's understanding instead of developing your own).

If you have the time you could read the religions’ foundational texts (Bible for Christianity, Quran for Islam, etc.) before commentaries on them.

I'm not surprised. There are a fair number of people here who are somewhat hostile to religion. Heck, you could borderline count me as one of them. I think religion is a blight on our world. But I wouldn't downvote somebody for mentioning a religious text in this context because - like it or not - these works tend to be HIGHLY influential, and if you want to "understand the world" I think that understanding absolutely has to incorporate some knowledge of religion and religious beliefs. I'm an atheist myself, but I have a whole shelf full of religious texts, including the Quran, several Bibles, Bhagavad Gita, Gnostic Bible, the Norse myths, etc. for just this reason. I doubt the historicity and literal veracity of a lot of this stuff, but knowing it has value when many other people do take it all literally.

Thinking in Systems by Donella Meadows Getting to Yes by Roger Fisher and William Ury ARRL Handbook for Radio Communications ARRL Antenna Book American Heritage Dictionary, 5th Ed. Bible/Lectionary, Breviary, and Catechism If fiction counts, I will probably never delete Flatland or the complete Joseph Conrad from my Kindle.