Make a secret compartment book

My inner Nancy Drew sqee-ed with excitement when I saw MAKE’s instructions on how to make your own secret compartment in a book. The Good Weekend magazine this past weekend had a story on former Columbian drug lord Pablo Escobar which had a photo of such a book, hollowed out to conceal a gun. But we’re geeks not drug lords, so let’s hide something less violent and illegal – what would you suggest?