That's a tough one. Artificer's Handbook. Ignoring the fact that I wrote the majority of it, I wrote it, because I needed it. To hell with the rest of you folks. It's *my* book.

Just kidding, of course, but it does help me figure out magic item costs, etc.
I've gotten a lot of use out of the Book of Templates, but I'm not sure it has my life easier, per se. Made my games better yes, easier, not so sure.
I'm still waiting for Creative Mountain's randomly generated spell lists by class, level, and archetype pdf. That will make my life easier.
Traps and Treachery 2 ( I don't have 1) has great traps that I can plug into a dungeon. That has made life easier.
I intend to use Philip J Reed's book of unusual treasures for a treasure vault. So, that will make my life easier.