Several years ago, my sister-in-law gave me a little do-dad which was designed to make it easier to hold a paperback book in one hand while reading it. It looked like something I ought to try to make myself, so I did. Here it is.
It is not a complex design, mostly pieces hulled together and using difference() to remove some parts. Check the SCAD file below to see how you might modify it to your needs. I do not know if the original do-dads are still for sale.
Extra kudos to you if you have read and enjoyed the book used in the illustration above.
A single print took about half an hour.
Available Files:paperback-assist01.scad - for study and modification, you will want this file paperback-assist01.stl - for printing as-is