As I have been writing book reviews and pondering what if anything else that is related to the topic that could be talked about. I know one debate is what is considered canonical in the GW universes. The debate centers around what GW considers to be the three view points extant: GW studio releases are primary sources, Black Library fiction is a secondary source and then their is (cringe) fan fiction.
For all intents and purposes, the GW studio only offers crumbs to the fluff hungry. Mostly in the form of unit descriptions, event time lines and short fiction revolving around key historical events that characterize an army. There are a few fictional excerpts or short pieces in army books and codeci as well, but they are often just snippets from novels or summarizations to whet the fluff minded's appetite. What is offered here (at least until the next edition of a codex or army book) is 100 percent canon.
The next lens is the author's writing. The text created and edited by Black Library and published for readers to drool and salivate over (at least that is what they hope for). Here, there is lots of different interpretations of the way things work, lasguns that shoot bullets for one thing comes to mind. The writing should not be confused with the author's intent, those are two different things. One should read a novel and interpret it on one's own because once it is published, what the author thinks or intended isn't necessarily true. Author interviews provide just another interpretation of events in a novel. That being said, most BL novels are fairly straight forward. I would also probably lump in the Imperial Armour books in with the Black Library books. One thing I noticed with IA 10, the short story C S Goto wrote "The trial of the Mantis Warriors" is completely ignored and most likely over written so they could present a version of the Badab War that Forge World wanted.
Finally there is fan fiction and fan interpretation. Just because it is on a website, blog or home made codex does not make it less valid (though the quality of writing could hurt the cause). Bolter & Chainsword created a great piece of alternate history fan fiction just by swapping Horus and Dorn's roles in the Heresy. It is a pretty good piece of work, and those unfamiliar with it should check it out.
In conclusion, if you have managed to make it this far, Games Workshop encourages its players to take an active part in the hobby. Don't feel that the story you might want to tell is invalid just because they don't care about reading it. The universe is vast, even in the 41st millenia, have fun, and remember that is what it is all about.