Ever notice how the title story in short story collections is never particularly memorable?
Not that this one is bad - it was a fun/fucked up little idea, and he got more out of it than I'd have expected, but it is clearly a lesser work.
Zoo is the simple tale of a man desperately searching for the bastard that killed his girlfriend...while fully aware that he killed her himself. He just can't admit it. So he pretends to search; every day he finds the evidence of their final trip together in his car; every day he follows it to the cabin where her body lies. He then pretends to remember the truth, pretends to reel in shock, and lies to himself that he will turn himself in. Then he takes a picture of her rotting corpse, and puts it in his mailbox. In case he forgets again.
Then he wakes up in the morning, finds the pictures, pretends to wonder who could be doing this, scans the picture, opens up a movie making software, and adds it to the end of the reel depicting her corpse as it rots away, twelve days a second.