It's a Sci-Fi Comedy about a superhero called Nuklear Man and the crazy adventures he leads (and believe me, he leads a lot of crazy adventures, the book's about 652 pages long). Every major plot area of the book either ends with something really funny or some awesome kickass fight scene.
The first two thirds of the book were crazy and unpredictable. The end got really serious and dark. I think Brian just wanted to show that he was more than a one-note writer. He wanted to show he could write something serious and something funny and off-the-wall.
Well, in the end of the book, he explains why it gets so dark. He essentially wanted to make this book the ultimate joke, and one night, he realized what makes jokes funny: Unpredictability. The best joke, therefore, has no punchline (as is what happens in the end of the story)