Just finished scanning and converting my first book on my 2nd-generation DIY scanner, and thought I'd pause for a moment to share what I have. I grabbed ideas that people posted, and had a few ideas of my own, and I'm satisfied with how it all came together.
Someone who had previously developed a glass-platen design on this site had a dowel between the glass panes, at the vertex of the platen, to keep the sides from "spreading" at that the bottom. I hadn't considered that might be a problem before, but realized when I thought about it that it probably would be. At the same time, I wanted to avoid anything that might interfere with capturing an image all the way to the edge of the glass.
My own solution was to build a frame around the platen, that holds everything together and allows the platen to stand free on the floor (which is where I set up). This is what the whole thing looks like, in action.
As you can (hopefully) see, I still haven't gone with preview monitors, but the LCD displays on my S5s can be rotated to give me a little real-time feedback on framing. It's necessary for me to watch these displays, too, at least until I can figure out if there's a way to turn off the "powersaver display off" function, because when the display goes off (even though the camera is still on), the manual focus goes off too, and it doesn't come back on until I turn the camera off and on, to re-load my custom settings.
These cameras allow only one custom setting (unless CHDK expands that too, I don't really know), so when I start shooting a book, I position the camera on the platen, frame the image (zooming as necessary), set white balance, manual exposure, and manual focus, and save the custom setting. The same setting SHOULD be good for the whole book.