Gather your materials - I found my book at the $1 section of Half Price Books. If you don't have Mod Podge, just break into your school supply stash and water down some Elmer's. You don't have to have a metal ruler, but I find it easier to use with the Xacto knife... personal perference, I guess.
Decide where you want your "box" to open and lay a piece of wax paper in between the sheets.
Brush on some of your glue to the pages keeping the book shut. The wax paper will keep the opening from being glued shut, so don't worry about that. Once you've coated all 3 sides, lay the book flat and place something heavy on top while the glue dries.
Once your glue is thoroughly dry, remove the wax paper. Using a ruler mark off the parts you'd like to cut out (I did a 1/2 inch border around mine).
Using your ruler as a guide, simply use your X-acto knife to cut out the middle section (make sure to not cut past the corners).
Remove the section you just cut out.
Continue cutting until you've gotten your box as deep as you'd like it. I found it easiest to use the utility knife so I could cut through multiple layers at one time.
If you have rough sections, you can do some light sanding to even them out, but it's certainly not necessary. Now take your white glue and paint it along all 4 sides of the inside of the box. Again, lay something heavy on top while it dries.
And, viola... you're done. Perfect for hiding jewelry, remotes or even spare cash that you don't want anyone to know about… sneaky you.