The Scattered and the Dead (Book 0.5)

Loneliness drives an agoraphobic shut-in to write a letter to the girl in the apartment across the hall, trying to strike up a friendship. Unfortunately, a series of apocalyptic events interrupt this attempt at human contact. Now he watches out the window as the world gets cut to pieces by plague and riots. There are even rumors of zombies. Getting to know someone could be harder than he thought, let alone surviving in a post-apocalyptic wasteland. He might even need to leave the apartment.