It's no secret that All Flesh Must Be Eaten was made for one-shots. While the core rulebook certainly does have more than enough information to run an extended campaign, it doesn't have specific rules for all of the problems you'll encounter in the long run. That's where this book comes in. It has rules for scavenging, jury-rigging, and farming. It also has new Miracles for the Inspired. The Deadworlds seem shorter than in other AFMBE books, but still contain useful information for your game. One of the Living has no short supply of items to use in your game either. Chapter 4 has items like iodine tablets for water, spear guns for killing zombies, and even Necronomicon Ex Mortis: The Book of the Dead.
Unlike most other AFMBE sourcebooks, where if you take out the zombies you have yourself a genre book, One of the Living works best for survival horror campaigns. That is actually what makes it my favorite AFMBE sourcebook. It feels more like an extension of the corebook than any other book out there.