[Sam Baughn]

I think the single most useful book for you after the basic set will be High-Tech. It's an excellent gear catalogue.

For really good survival and scavenging rules, get After the End 2 (AtE 1 has some character stuff and improvised weapons which might be useful, but book 2 is the really helpful one).

Detailed combat options from Martial Arts and Tactical Shooting could add even more detail, but the basic system has plenty of detail already.

Zombies has a load of stuff about zombies, although a lot of it is advice on different genres to use zombies in, alternative zombie templates, etc. which will be less helpful if you already have a strong vision of exactly what you want. It does have rules for things like managing zombie hordes with a reasonable amount of dice rolling and a slightly different improvised weapon list to AtE1 (there's a lot of overlap though; if you want even more improvised weapons, you can find them in Horror and Martial Arts).