I'm gonna throw in the Codex Imperialis.
Just be aware that every source you look at may, and likely will, present you with
different answers to your questions. This chance increases once you move away from
GW material and delve into the licensed stuff like novels, as it's usually written by other people, and since
40k cares little for canon and consistency everybody tends to throw in his own ideas. Needless to say, artistic license will thus create a multitude of different interpretations. Feel free to "adopt" whatever you like - or come up with your own stuff as you go.