Open the book to a page approximately one-third of the way into the book. For example, for a 500-page book, open it to about page 150.
Cut a square into the pages of the book using the craft knife, using our example, from about page 150 to page 350. This may require several passes of the knife across the same surface to cut deep enough into the pages. Make the opening as wide as necessary for the amount that will be hidden in its pages.
Place the money into the opening in the book.
Close the book and place it with other books on a bookshelf, ensuring the hiding book is recognizable by the people who need access to it.