Hold your books upright, and put some gridfinity bins of your choice on the outside.  This is printed in PLA and holds a shelf of books just fine.

Dimensions:

• The model is roughly 130mm wide by 160mm tall.
• The inside base (the part that goes under the books) is roughly 80mm deep and 1.4mm thick, which seems to be enough to hold.
• The upright arch is 4mm thick and has no problem with a shelf of books.
• The outside base (angled back) just fits a 1x3 gridfinity base.
• The elliptical cutout has only a small amount of overhang at the top, so it can print without supports.

How sturdy is it? Plenty!  When pushing books against it, the PLA starts to slide before the upright arch is close to bending or breaking.  A thin non-slip sticker on the bottom could help, but I'm not going to bother.  For standard use with 40cm – 50cm of books, it's just a non-issue.

There are three stl files:

• the bookend, with an empty space between the angled fins,
• a simple 3x1 gridfinity base, and
• a simple 3x1 gridfinity bin, divided into two compartments.

You can use the included base, or any other 3x1 base/bin.  Combine the pieces in your slicer to print them as a single unit.  I printed the outer frame and base together, so that I could swap out bins later.  The included 3mf file has that combination.

Freecad file included for further modification.