When all is said and done, and it all boils down to a single word answer, that answer is...
Justin was thinking of the Chtorr series by David Gerrold.
Y The Last Man is a good post apocalypse story. You should read it.
An interesting twist on the Alien invasion Post Apocalypse would be if after Humanity set up giant space empires, War breaks out and the Earth is fucked and there by turned into mostly deserts and stuff, and go from there with little to know news of what happened outside of Earth.