What does it do exactly and what book do I find it in?
Thanks in advance
DogBone
From the SRD:
Splash Weapon Mastery
You are adept at increasing the range, area, and accuracy of splash weapons.
Benefit: When throwing a splash weapon, you act as if you had the Far Shot feat. When you hit with a splash weapon, select one additional square adjacent to the splash area; creatures in this area also take splash damage. When you miss with a splash weapon, you may adjust the miss direction on the grid by +1 or –1. This feat counts as Far Shot for the purpose of qualifying for other feats, but only in regard to splash weapons.
Source Adventurer's Armory 31