Step 13: Bookholder: Introduction and Design Issues
One of the nastier problems in scanning books is that the "center" of the book shifts as you flip through the pages. I have a couple pictures with a very thick book that demonstrate this problem. Essentially, with books that have any thickness, the center "V" shape of the book moves left to right as you flip pages. However, the platen of our book scanner does not move. That means that we need whatever is holding the book to slide freely, to center it below the platen.
This is the main reason why the base of the book scanner must have a smooth surface, like melamine, plastic, or counter top. Things have to slide.