I wondered where the illusion of the second moon had come from, but I only wondered for a moment, and then I dismissed it from my thoughts. Perhaps it was an afterimage, I decided, or a ghost: something that had stirred in my mind, for a moment, so powerfully that I believed it to be real, but now was gone, and faded into the past like a memory forgotten, or a shadow into the dusk.
Does this guy have a way with words, or what? Aside from the beautiful prose though, the ending is hauntingly vague, leaving you with that feeling that you get when you wake up suddenly and can't quite remember the dream you were in the middle of having. And this last paragraph (like a lot of the book, actually) has two possible interpretations: what you get on the surface, and the slightly more in-depth understanding of what Gaiman meant to convey.
The first time you read it, it sounds like the guy has just mistakenly thought that he saw two moons in his rearview mirror when driving away from the farm. He's an adult, though, so he knows there's only one moon, and it'd be silliness to think that he really saw two. So he shakes off the impression of dual moons like you would if you felt like you just saw someone you know when you're very much aware that they are far away and can't possibly be in the area. You might shrug, or shake you head and laugh it off.
Read a little deeper, though, and you might find that Gaiman is referring to the whole story that we've just experienced. Are there really such things as hunger birds, and fleas, and three Hempstock women capable of extraordinary things? Did Gran and Ginnie really wipe his memory to protect his fragile human mind, and is that what's leaving that feeling like a remnant of a dream behind? Or has everything been a fantasy created by a lonely boy—"so powerful that I believed it to be real"?
Think about the words he chooses: a memory forgotten, and a shadow in the dusk. If a memory is forgotten, does it exist anymore? Do shadows even occur during dusk? If a tree falls in the woods and no one hears it, does it make a sound? The more we think about, the more we're really not sure what's up with the ending.