Some people consider it a book safe, as a way to sneakily hide belongings in plain sight. In this case, it would probably be ideal to use a hard cover book with the outer sleeve still on it, just to make it look more natural.
For me, I chose a book with a clever title to make a box for my dad. It was a computer joke, and since my dad and I both work in software, I thought it would be a perfect gift from me. He can use it as a box on his desk to hold notes, business cards, office supplies, whatever. Or, he can go the safe route.
It was pretty simple, I just used an X-acto knife to cut out the center of the pages all the way through. It was a bit of a slow process, only cutting through a few pages at a time, so when you try it at home just sit down with a movie and cut cut cut. By the end of the movie, you should be done!
I then cut the scrapbook paper to line the inside covers of the book, to give it a finished look. The last step was to glue the pages together to form a firm box. You want to make sure to glue the back of the last page down to the back inside cover, and then the rest of the glue simply goes around the three edges of the pages. I brushed on two layers of Mod Podge with a sponge brush.
The last trick is to then wrap the inside front cover (top of box) in foil or plastic wrap to prevent any of the glue getting on it, close the book, then pile other heavy books on top of it until the glue is dry. This will ensure the box is firm. When you remove the plastic wrap, if any glue seeped up on the front of the top page, use a fine grit sandpaper to gently remove it, so that the top page retains the papery quality of the book.
I put a gift card inside the book before I popped the gift in the mail. If he doesn't want a trinket box or a bookshelf safe, he can throw it away since it only cost me about $1.50 to make, since I bought the book from a thrift store and only used one sheet of scrapbook paper to line it... you can hardly buy a boring gift box for $1.50 anymore these days anyway!