The Old Hollowed-Out-Book Trick
If the sides of the books will be visible, fold back the covers of the books on the left and right sides of the assembly before cutting. Build a plywood box for the secret compartment to fit the opening and glue the book parts to the box with construction adhesive. The disadvantage to this hidden compartement? You can see inside the box on low shelves, so you need to display it so the opening is above eye level.
Right Out in the Open
Cut out a stud space opening to fit a return air grille. Cut off the grille screws and glue just the heads in place. Run four drywall screws into the corners of the opening so they fit just inside the rim of the grille. Then glue rare earth magnets to the back of the grille so they line up with the screw heads.