So here's the question: What title would you use if you were trying to pass your own book off as an old classic fairy tale? You'd probably pick something that sounds really old and… classic-y. And The Princess Bride fits the bill perfectly. Plus, Buttercup's at the heart of the drama in this one, and she's, well, a princess bride. So we're tipped off to this, too, by the title.
But before we start high-fiving about how easily we sorted this title out, let's make note of the fact that the actual title of this book is The Princess Bride: S. Morgenstern's Classic Tale of True Love and High Adventure… by William Goldman. And this full title sets up all of the drama around narration. This is basically a book within a book, after all, with a fictionalized version of the actual book's author amending another fictionalized author's work for his own purposes. It's a bit of a brain twister, and the complete title gives us a hint of what's to come in this regard.