Is there any place I can read / watch that I can find-out something more about the Angels. I just finished watching Rebuild from 1.0 to 3.0, and it looks like everyone calls these strange beings "Angels", but why is that? Where are they coming from?
Why are the angels called that way ?
The answer can be found here:
Because Christianity is an uncommon religion in Japan we thought it would be mysterious. [...] We just thought the visual symbols of Christianity look cool. If we had known the show would get distributed in the US and Europe we might have rethought that choice.
Note: This answer is based on the original series, it's accompanying movie and printed materials, and Rebuild 1.0 & 2.0.
No one truly knows, or if they do it was never disclosed in any of the official materials. All we know is that they are beings with tremendous power that have DNA that is %99.89 similar to human DNA. It is also stated that humans are descended from the Angel known as Lilith (the white giant imprisoned inside Terminal Dogma) while the sixteen Angels that threaten humanity are descended from the Angel known as Adam (whose only remaining part is inside the brief case Kaji delivers to NERV). Also not known is the source of their naming. Were they the source of the stories of their biblical counterparts, or were they named after the fact using the Bible as a guide? Though personally I tend to lean towards the latter.