Hello,
I went with other rules myself, so I don't have ATZ, but as I've been told, ATZ:BDTZ is enough. Or at least, enough to start with. Haven is book with scenarios and I, zombie introduces some changes to the play and other zombies, like ragers and smart ones. If I am correct, there is also a free PDF for some big worm thing, if you're in that kind of monsters.
Hope this helps,
Mathyoo