[spoiler]There isn't any.[/spoiler] I believe that the amount of story that they included into the game is what they were going for right from day one. They want us to read the grimoire and share our experiences on these forums. Find those little bits of information locked away and guess. We currently live in the most social open world, where communication is at the push of a button. Twitter, facebook, reddit just to name a few. I believe bungie had this in mind knowing that we would interact and share our experiences about the game. Look at how many other games have stories laid out within their game and still people invent prologues and epilogues. Also I wouldn't want to sit through longer extended cut scenes more then I already am. And before you ask, I'll answer it. How come we can't access the grimoire from in game? Because you can't access Twitter, Facebook or reddit from in game. But, you can from you cell phone. People are just used to games being one way and when someone changes the curve everybody gets scared. Is it for the better or worse? Time will tell.