I'm constantly on the lookout for visually interesting discarded texts to excavate. I work slowly from the back of the book picking out images to keep. I use bookbinding adhesive to affix acid free cardstock to the back of the selected image, then remove the unwanted part of the page with x-acto knives. The end product is a collage of images and/or text which had always been there. I just assist in revealing them (although I sometimes tweak an image or two to benefit the overall composition).