League of Legends (LoL) Item: Sorcerer's Shoes

If you aren't buying them as an apc...you aren't playing Apc.

good build

No, it will not reduce further than zero. If they had zero, however, and you had dread from fiddlesticks which reduces magic resistance by 10, that would reduce them below zero. Here's a page that explains how it all works.
http://leagueoflegends.wikia.com/wiki/Magic_penetration

If you have Sorcerer's Shoes, and the opponent has no Magic resist, do you do 20+ damage??

These shoes are great for any ap carry!

It means all your ability power will go through 20 points of their magic resistance

If your opponent has 20 magic resistance, Sorcerer's Shoes will treat your opponent like they had 0 MR. If they had 40 MR, it treats them like they only had 20.

the 20 magic pen... does that mean 20% or that 20 damage will always make it through their magic resistance?