Cut out two pieces of masonite hardboard the same size as the pages of your book to use as a template..
Mark the outline of the compartment on your template. Free the back and front cover and a few up front pages and now clamp togethr the rest of the pages between the two template boards.
Drill a few pilot holes, and add a few screws thru both boards to keep the book pages together. Drill a pilot hole to use as a starting point for your jigsaw and then start cutting out around the guidelines of your template.
Once that's done the center will come out easily.
Place a sheet of wax paper inside the back cover?and another protecting the front cover and extra pages while you spread glue around the inside.
With a 80/20 mixture of white glue and water brush around the inside edges of the secret comparment? You can also brush some glue on the outside edges for extra bonding once thats done, remove the wax paper on the back cover and glue the back page to the back cover? Clamp the book overnight to dry.
These also make great gifts? You can pick out a book to suit the personality of the person you're giving it to! I'm Mrs. Fixit and it's just that simple!