Friend or UFO

I found this book at my library (who's second floor is under reconstruction after a flood, and the second floor is where all the grown-up books are). It's the third book in a series, but I found it in the new releases, so I just jumped into the story. It was fairly easy to catch up, but of course there are a few things I don't fully understand. But it was a fast-paced, fun story full of humor and imagination.