Here is a bookend with a cast iron pipe design. This is modeled after the popular bookends as seen on Amazon and Etsy.

Parts

1. Pipes: I used Sunlu black PLA. Supports are required for this model to print properly. I experimented with a few settings and I got the best results with 2 tree support walls for angles over 70°. Mess with support Z distance (I used my layer height) and support X/Y distance (reduce until supports are modeled under pipe elbow fitting). I also printed 3 walls for the entire model and used fuzzy skins to give the pipes a rough cast iron texture (0.1 thickness/5.0 density).
2. Stand: I used Hatchbox wood-filled PLA and printed on its side (for strength).

Assembly

Glue the pipe to the stand as shown in the photos. You may need to add little rubber bumper feet to the bottom of the stand to prevent slipping on the shelf depending on your bookshelf material.