This is a zombie book, so we're all about the brains up in here. Even if you haven't read a page of this book, you can guess as to what these brains might represent: memory. When a zombie in Warm Bodies dines on a juicy hunk of human brain, they have access to their memories, for just a moment.
This quick glimpse of memory is like a drug to a zombie. They have no memories of their own, and they get addicted to this memory rush. In one scene, R and M pass a brain back and forth between each other, taking a bite and pretty much tripping on the visions that result. The brain is the ultimate hallucinogen.
By doing this, the zombies are doing the opposite of what live humans are doing. They are actually putting these brains to work while we Living seem to try our hardest to turn them off. Oh, the cruel, cruel irony. When talking about drug use and other self-harming activities, Julie says, "All the shitty stuff people do to themselves... it can all be the same thing, you know? Just a way to drown out your own voice. To kill your memories without having to kill yourself" (1.7.60). It's the zombies who are bringing that voice back to life, as strange as that sounds.
Finally, we'd be remiss if we didn't point out all the brain puns that Marion works into the text. Let's just say that the phrase "pick his brain" (1.2.3) ends up with an entirely new meaning.