Sure, we've all heard about the book safe trick where a hole is cut into a book to hold money or hide rock hammers from a prison warden. This DIY booksafe from Instructables user craftknowitall ups the ante by creating a hidden storage box whose size is only limited by your bookshelves and how many hardcover books you want to sacrifice.
You'll need a sturdy cardboard box, a table saw, blue painter's tape, and white glue along with E-6000 glue. You'll also need to have a measuring tape, box cutter and sandpaper. If you have the tools your only real costs should be five bucks or so for the E-6000 glue and maybe the painters tape.
Remove any top flaps from the cardboard box and make sure you have enough hardcover books to cover the side of the box that will face the front of your shelves. Wrap all the books with tape to make sure the pages won't tear as much when you cut the books and then cut the spines from the books with the table saw. Apply white glue to the surface of the book pages cut by the saw to make the pages into a solid mass and let the glue dry.
Finally, remove the painter's tape from the book spines and, one by one, glue the books to the front of your cardboard box. If the cardbox box has a glossy surface you'll want to rough it up with the sandpaper so the glue will better adhere. Let that dry and you'll be ready to fill your new box with valuables and stash it amongst your books.
This should only take around a half hour if you have all the materials on hand. Make sure you check out the source link below for more detailed instructions and step-by-step photos.
How to Create a Secret Bookcase Box [Instructables]