Let me see if I have all of this straight after tonight's episode.
There was never a switch of cigarette packs. There was never need because Walt tricked Jesse into thinking the ricin cig was sucked up by the roomba. The cig has a vial of salt in it because he wasn't going to give it to Jesse...
Actually, Dave from last year, if you go back during last season season, when Walt is moving back in with Skyler, he is unpacking everything and he finds the book. He stares at it, laughs for a few seconds, and places it on the toilet. He thinks he's beyond reproach at that point.