In essence, such a scanner is a mounted camera taking photos of a well-lit environment.
Originally, such scanners were expensive and could only be found in archives and museums, but with the availability of cheap, high-resolution digital cameras, DIY planetary scanners have become affordable, and for instance are being used by volunteer scan providers for Project Gutenberg.
They also require books to be fully opened most of the time (there are some exceptions where the scanning surface ends at the edge of the flatbed scanner, so that a book can be opened partially).
Many of those scanners are equipped with self-balancing book cradles, gaps for the binding to be placed in, and pressure-sensitive glass controls.
that by opening the book to a full 180 degrees and using scan glass, the scanner is able to capture further into the binding than those using a V cradle.