Max Brooks' Zombie Survival Guide is a must-buy for any self-respecting zombie connaisseur. It's go a lot of useful information, most of it concerns only a specific brand of zombies (slow ones) but it is such a great tool for both GM and players alike that you should definitely look into it. At worst, a zombie game can be run using any system that doesn't let you chop their head off by dozens with a katana. (Unless that's what you want.)