When I used a camera, I would place something lying horizontally across the book (remote control or something) to keep it open. Obviously, that's probably going to obscure some of the page and would necessitate taking two pictures if text was covered before.
I've tried scanning books by putting something heavy (massive Spanish dictionary I keep near my scanner) onto the spine to push it down to the scanner, but it still doesn't always seem to work. The only sure-fire way to get a good scan would be to unbind the book (cut the pages out, melt the glue, whatever).