My monster book is finished, and if I'm allowed a small self-congratulatory moment, looks pretty terrific! It takes an astonishing amount of pressure to sew the leather onto the cover, so if I ever do it again I'd definitely try simply sewing it to the paper, and then just glueing the paper to the cover, but I'm not sure how to get the placement exact enough to make that work. This way I was able to stretch the leather a little bit, and it fits very nicely over the book.
Here are a couple shots of the terrific paper I used for the endpaper. The anatomy poster looks great, but it was pretty thin, so I had to glue it to another sheet of paper before I used it, or ever single bump from the stitches would have shown through. I put the page at the back sideways because I wanted to continue the whole 'random recycled parts, use what's available' theme, a little like the way Doctor Frankenstein made his monster.