I haven't read "WWZ" nor "I am Legend". Based on the comments some of you here has made about them I am more or less obligated to pick them up and read them.
In my opinion there is not such a big gap between zombie fiction and post apocalyptic fiction. The zombies are just a threat among others. It's the interaction among the surviving humans that strikes a chord in my mind. Humanity at its worst.
The Endworld series of pulp fiction written by David Robbins is a good post apocalyptic read. A group of survuvalists, or descendants to survuvalists lives in a bunker built before WW3. The threat is those books aren't zombies, but other humans and radiated mutated animals mostly. Still a good read. Those book were even translated into Swedish back in the eighties.