Can anyone explain to me mastery books and simulators?

Mastery Books are Fourth Job items that allow you to use more Skill Points in a specific skill (increasing the mastery level to 20 or 30). They also work on a percent basis so they can sometimes fail, such as a scroll would.

A stimulator (I assume is what you meant) is an item used when forging weapons. I'm not sure exactly what they do, as I've never used one. The description is "Item option changes when you make [weapon] with this stimulator. However, creating item may result in failure by 10%" I assume it changes the stats of the weapon to something other than average.

Stimulators change created items from having normal stats to either above or below stats.
It's like gambling. Plus a destruction rate.

Simulation is like having the item dropped from a monster, it can be godly or crappy or not even come out at all.