Horne is a graphic designer, and he makes each end-of-the-world visually appealing in its own way. (This blog post has tons of photos of the interiors, if you're curious.) Each threat comes with a grading system of when you should start to panic (Later, Too Late, Soon, or Now), a brief description of the peril, and bits of related information. This isn't an in-depth encyclopedia or anything; it's a book to be flipped through, a mix of an art book, a humor book, and a collection of trivia.
Some commenters have argued that I'm too hard on humor books, that I'm grading frivolity on the same scale as "real" literature. This book is a perfect example of how you can do a humorous book in an interesting, worthy way. Horne clearly put the time into making A Is for Armageddon a real visual pleasure; it's not just a rehash of a blog churned into book form. It's an attractive object that entertains.