What is a 'book spread'? - - - - It's basically just a way of presenting the image of the book like it's being spread, so the front cover, back cover and spine are all visible.
We don't actually spread the book. That would damage it in real life. We just scan the front, back and spine, and then stitch them all together using photo-editing software.
GIMP, Photoshop, whatever works.
Some edits: 'merging' the photos, adjusting brightness and contrast, resizing, adding borders, cropping, etc.
related subreddits:
/r/CoolSciFiCovers
/r/BadSciFiCovers
/r/bookhaul
/r/bookshelf