You mean you're still craving more brain snacks after reading Warm Bodies? That book is filled with them! Because R is a zombie. He eats brains! Brain snack, get it?! Har har har... Okay. Moving on...
Isaac Marion wrote a short story prequel to Warm Bodies for City Arts magazine. It's set at Christmastime, and Marion calls it "significantly less heartwarming than most holiday specials." Heartwarming, no. Brain warming? Definitely.
Warm Bodies started as a short story called "I Am a Zombie Filled with Love." It's a little different than Warm Bodies, but you can see how Isaac Marion filled this story with love and turned it into a full-length novel.
Not enough prequels for you? There's another one: "Grass through the Concrete," which stars Julie, Nora, and M. Want to read it? Too bad. As of January 2013, it's out of print. Better find a friend and borrow it. (Source.)