I've done
one book, one because I'm pretty sure it being digitized isn't going to happen for a long time (if ever) and two because I wanted to go through the whole scan/ocr/proof process to have some experience with how things worked.
It was a bit tedious as I didn't want to destroy the pbook so I did each page on a flat bed, but it wasn't too bad since I wasn't in a hurry (I just did a handful of chapters on my lunch hour each day until I was done). For OCR I used the Finereader engine that came with the aging Microtek scanner I worked with (didn't want to buy the full version for a one time project/experiment) and was surprised how well it worked. I checked the text against the physical book page by page and the whole process scanning, OCR to final ePub took about 8 hours.