It looks like just a book on your shelf . . . but when you open it up there is a hidden compartment inside!
To start find one of the most boring, hardcover books you can at a secondhand store or thrift store.
Separate the first few pages from the others with wax paper
Paint watered down glue around the edges of the book and weigh it down overnight
Next open the book, leaving the wax paper on the first few pages, and draw lines a ruler's width from the outside edges of the page all around.
With a craft knife - and adult supervision - cut along the lines of the rectangle formed. This part takes a long time and is quite tedious. Don't try to remove more than a few pages at a time and Mom should be ready to help about a quarter way through the book.
When you have reached a few pages from the end of the book, stop and paint glue along the inside and outside edges. Release the wax paper held pages and put the whole book under a heavy item again overnight.
The next day open the book and cut through the page that stuck to the others carefully and neatly. This covers up the pencil marks and any other little mistakes. Fill it up with your treasures and hide it on your bookshelf!
Or, if you just cannot bring yourself to go to all of this trouble I have seen them for sale on Amazon