My daughter asked me for a book stand and I took http://www.thingiverse.com/thing:3643872 as a starting point, but transforming it on a single part with Blender, so that it can stay fully flat on a table. I also removed back pillars, reinforced the back central support and added a back extension for stability.

If you feel it is not stable enough you can increase the length of the back extension.

21/01/2021: Updated to increase the resistance of the front pillars.