Hidden in the front and back covers are 3-D "magic" stereogram images. The invisible images reflect the philosophy of the book---That by exploring the numbers closely, rich and unforeseen structure and nuance are revealed.
Viewing "magic" stereogram art is done by looking through the image, allowing the pattern to overlap visually as your eyes focus past the surface. Here is one method to help see the 3-D images.
Hold the book right up to your face so your nose is touching the cover. Now imagine that you are looking straight ahead, right through the book. Slowly pull the book away while keeping your eyes pointed straight ahead. If you do this slowly enough, an image usually appears when the book is at the correct distance. It may take some time before the image appears--just as in mathematics, you must be patient.
If you are interested in what is between the covers, you might want to read Fernando Q. Gouvea's review at MAA Online.
Can you see the hidden detail in the number jungle?