
One of the more "head-scratching" signature weakness, it definitely plays to Norman's "flavor," both mechanical and fictional. Looking at the two elements, the effect and the discard condition, we get:
The effect: This locks Norman's deck, preventing anything that would interact with the deck, so no drawing, searching, or anything else. This is generally pretty bad for a like Norman, although, if it comes out very late in a scenario, it can be ignored as you race to the finish. It's kind of the reverse of Roland and Jenny's weaknesses -- if you see those late, you are usually doomed, but, if they come out early they are more nuisances. Since messing around with the top of his deck is pretty much what Norman does, if this comes out earlier than the penultimate turn, you need to grit your teeth and clear it pretty quickly. Occasionally, you will get the mild satisfaction of seeing some encounter deck effect (like the Dunwich milling) fail, which is fun, but the sooner cleared, the better. There isn't really any mitigation you can do.
The discard condition: Spend . Since it functions "as if it were in your threat area," another investigator can clear this in a pinch.
All in all, this is an average, maybe below average weakness. It's not going to cost you the campaign, but Norman could be defeated if this got drawn while he was low on cards and in combat.
Box vs book This is a bit more persistent than Vengeful Hound (which has a similar effect, plus biting), but Livre d'Eibon is better than Split the Angle, so the "box" set sees more use, although you can run both without too much trouble.