Ah. No, I don't think the wizard in my group has ever scribed a spell into his spellbook beyond the two freebies he gets each level. We just haven't come across any other wizards' spellbooks, I don't think; nor have we really had time for him to go searching for other wizards willing to let him borrow their books.
Well, if you're planning on playing the adventure straight through, I imagine you will be spending the majority of levels 1-15 in the Underdark. That's sort of the point of the adventure.
If, however, you're just going to use bits of it in a different campaign, then maybe yeah. But if you're going to be doing that much customizing already, why not just customize the loot in the process? You can also always ignore the rule about drow magic not working in sunlight as well.