There's some story there in the game, but you really have to pay attention or it would slide right past you, and once you complete the main story-line in the Vanilla version of the game (not the DLCs) you are basically left wondering what just happened. So yea, the Grimoire is a huge story buff to the game. If you really want to understand the things that are happening, you almost have to read it. If you are just wanting to shoot things and don't care, I wouldn't bother.
There's some story there in the game, but you really have to pay attention or it would slide right past you, and once you complete the main story-line in the Vanilla version of the game (not the DLCs) you are basically left wondering what just happened. So yea, the Grimoire is a huge story buff to the game. If you really want to understand the things that are happening, you almost have to read it. If you are just wanting to shoot things and don't care, I wouldn't bother.