There's a ton of Harry Potter stuff floating around the interwebs this week - this one's a couple of months old, but it's still pretty sweet. Reddit user Trehlo hollowed out a book to hold the engagement ring he was going to present to his now-fiancee - but the tome was only partially carved out. As you can see, he stopped at a very specific chapter. Pretty clever.
Link via Geeks Are Sexy
The first chapter is "The Other Minister"
Chapter FIFTEEN is the 'Unbreakable Vow' (when Harry learns of the vow by eavesdropping on Snape and Malfoy) although that vow is made in the second chapter "Spinners End"