I have no issues with the game killing off the skill book vendor.
This is just poor design no matter how you look at it, unless you're an idiot.
This happens right at the specific point before a huge boss fight against the single toughest poss in the game, and you have no reason to wander off and check all other NPC's at that point.
It is absolutely bad design to have quests kill off skillbook vendors no matter how you look at it. This was severely obnoxious in Act 1, and they never changed that stupid decision then, despite complaints.
There is absolutely no reason for this to be in the game.
