Turns out, there's a book with the same name. And I think that it's a really, really fun-looking book, which means you probably shouldn't get it for me--let alone the Pillowfight Fairy.
However, the same author has written several other books about how to make homemade siege engines, including one entitled The Art of the Catapult: Build Greek Ballistae, Roman Onagers, English Trebuchets, and More Ancient Artillery. This one is probably a little less dangerous, because there isn't anything flammable involved. And given our five-year-old's demonstrated interests in this area (not to mention her dad's), it's probably something we'll wind up with one day. Hey, we've got a big backyard (though, maybe not quite big enough....)
(My wife just piped up with: "Well, it depends on just how big you build these things," to which I reply, "That's exactly the point!")
I also love the title of yet another of his books: Whoosh Boom Splat: The Garage Warrior's Guide to Building Projectile Shooters. Of all the projects in this book, the really fun-looking one is the "Mechanical Toe"--a bungee-powered kicking machine. It might not be particularly useful, unless one feels like one needs a good kicking on one's backside and there's no one around to administer it, but it would still be cool to have.
(My wife just mentioned that it might be useful if you were constructing a giant game of Mousetrap. Did I mention before that she's the practical one of the family?) :-)