NASA are extremely liberal with how you're allowed to use these images. There are other people selling prints of them, for example.

"Unless otherwise noted, images and video on JPL public web sites (public sites ending with a jpl.nasa.gov address) may be used for any purpose without prior permission, subject to the special cases noted below."

The "special cases" mostly deal with using the NASA or JPL logos, or using photos with real humans in them commercially.

http://www.jpl.nasa.gov/imagepolicy/

They do as far as I can tell by reading the license of the images. http://www.jpl.nasa.gov/imagepolicy/

Just bought the book. He has a few paragraphs on the cover image and states "Many thanks to NASA/JPL for allowing reuse."

IIRC government-produced stuff is all in the public domain.