First of all, parts of the cover glow in the dark. The illustrations were made using a technique called cyantotype printing. According to Redniss' website:
Cyanotype is a camera‐less photographic technique in which paper is coated with light‐sensitive chemicals. When the chemically-treated paper is exposed to the sun’s ultraviolet rays, it turns a deep blue color.If you're interested in cyanotype printing, the Madison Public Library is holding a few classes on it. If you're interested, you can register and get more information here.