[NTG-context] How can I design a book cover with front cover, back cover, and spine?

Let's assume that I'm writing an A4 book. How can I design front cover, spine, and back cover of the book with ConTeXt? I've read many PDF files, but none that I can remember actually had book spine on it. Some had back cover. Most had front cover. Do printing houses make spine and back cover for authors so that back cover and spine don't need to be typeset by LaTeX or ConTeXt?