This Bookend utilizes the load of the books and friction to secure itself in place. When unloaded it can freely slide along the shelf. This model is designed with probably the most ubiquitous book shelf in mind, the IKEA Billy. This shelf is 19.5mm thick but this bookend can easily accomodate thinner or thicker shelfs.

For increased friction in recommend to use a height range modifier to enable fuzzy skin but you don't have to.

To safe time I recommend using a 0.6mm nozzle and 0.3mm layer height. The GCODE files are sliced for that.

For the needed strength use 4-5 perimeters (0.4mm nozzle) or 3 perimeters (0.6mm nozzle) and 25% Gyroid or Cubic Infill.

I tested it in Prusament PETG but it should work just fine in PLA, too.

UPDATE: Added the mirrored left side version. Added V2 with increased thickness of brace for higher stiffness.