Reddit reviews Greek Religion

Since you mention wanting to learn about other neo pagan belief systems, I'll speak up for Helenic polytheism, which is the modern worship of the Greek gods, usually with an emphasis on reconstructing what the ancients really practiced.

Kharis: Hellenic Polytheism Explored https://www.amazon.com/dp/143823192X/ref=cm_sw_r_cp_api_b7E6BbE517CQB. This is the book I recommend giving to relatives who want to know who we are and what we practice in the modern age.

Greek Religion https://www.amazon.com/dp/0674362810/ref=cm_sw_r_cp_api_c8E6Bb542VQ1W. This is an academic text on religion in ancient Greece that is very useful both in learning what the ancients did and believed as well as a source book used by many modern practitioners as they adapt ancient beliefs and practices to modern life.

For a scholarly but interesting book about ancient Greek religious practices, check out Burkert's Greek Religion. For Norse religion, I'd just jump in and start reading the Poetic Edda.

Fortunately, there's still a wealth of original sources available regarding Greek and Roman religion, as well as Norse to a slightly lesser extent. Sadly, much of what's known today about pre-Christian Celtic religions comes from secondhand accounts of invading armies, or from the archaeological record, so it's not nearly as complete.

Yes, "God" and "a god" should be thought of as two entirely different kinds of being; or rather, the former is not "a being" at all, existing alongside other things in the universe. And in fact, in the Classical Era, many Greek philosophers did rationalise the traditional gods as abstract principles subordinate to Zeus, who was thought of, in a way, as the only god who really existed.

Further reading: On the distinction between God and gods, The Experience of God, pp. 28–41. On Greek philosophical religion, Greek Religion, particularly chapter VII.

Also one book I've been curious is this

http://www.amazon.com/Greek-Religion-Walter-Burkert/dp/0674362810/ref=sr_1_1?ie=UTF8&qid=1463239850&sr=8-1&keywords=greek+religion

it says it's good, but it was written in the seventies. Has there been any great development since then?

u/HideousRabbit is right: this is not a philosophical question. That said, a classic in the field you describe is Walter Burkert's Greek Religion (German original 1977, English translation 1985). A more recent work is Daniel Ogden’s Companion to Greek Religion from 2007.