On that particular point, I find it annoying that major events or significant motivations for a character's actions are not told in game and I personally won't look outside the game because I'm not that invested and because I only want to devote so much time and effort to what is, after all, a video game universe.
The thing for me is that we pay $50+ for the game and another $150+ for a year's worth of subs. Time investment aside (which is also significant) I think that for that price they should put the main beats of the story in the damn game. When I hear that "oh, everything that Character A does in game that seems to nonsensical in game actually makes a lot of sense when you read this short story..." it's annoying. I don't expect all of the detail of a short story or a novel in a cut scene, but I do expect that the in game scenarios and quests tell me enough that I get the main points of the story and the main motivations of a character if those are to be revealed at all. Books etc are find for fleshing out the world, telling side-stories and the like.
This is more than just me though - they should also recognize that many, probably most of the players of the game will never read any of the stories, whether they're freely downloadable or for purchase. For those people the game is the entire story and it should hang together and make sense for them.