NieR

This not missable and part of the storyline. When you get to the top of the Lost Shrine as your first foray into a dungeon, you will come upon a hallway with two large inactive robots holding out their weapons between eachother forming a circle around a book suspended in the air. This book is Grimoire Weiss. Simply go up and slash at the book for a bit (no special attacks or anything required) and eventually he will whimper and get shattered out of his barrier. Then, when you have the option to ask the book for help, do so, and he will have joined your party, and you will begin to gain access to some of the freaking badass magic in the game. Have fun!