First you will need:
a large book
two more heavier books
First, using your paint brush and glue, open the book to the middle section. Glue around the edges of the books pages. Do this on one side of the book, the side that will be hollowed out. Make sure there is plenty of glue around all the sides and that the pages are covered in glue.
Now, take the card board and fold in half and place in the open section of the book. This will keep the two sections from sticking together. Close the book and place two more heavier books on top. Let the glue dry for 30 mintutes to one hour.
Make sure none of the pages are lose after an hour. Is they are, add more glue, compress with the other books and let dry again.
Now that the glue is dry, measure in the center of the book with the glued pages. Measure inside the perimeter of the book. Then take the exacto knife and begin cutting the square out of the book. You will only be able to do a few pages at a times. Continue cutting until you have reached the middle section or so of the glued book pages.
Next, glue the inside of the pages. Again, make sure there is enough glue. Place the cardboard in the center again and the two other heavier books on top and let dry.
Your hollowed book is done and ready to have items placed inside.