I made this stylish organic-shaped Bookend a while back and figured I would upload it for everyone to use.
It should be plenty strong to keep your books from falling over, it has worked for me for months.

SETTINGS

important --> the amount of bottom solid layers should be at least 5.

I printed it in regular PLA, you can set the layer height as high or low as u want, just keep in mind the quality you desire. (I used 0.24 and it looks great).
I would recommend using the paint-on support feature to put a little bit of supports in the holes to make a cleaner print, it will work without supports too but requires more work cleaning up after.
Infill doesn't really matter as long as its 5% or more, I have printed one in 5% and 3 perimeters.

optional -->If you want to increase the strength you can increase the amount of perimeters on the first 5-7 layers.

EXPERIMENTAL

I've been thinking about pausing the print halfway and adding sand in the infill to make it heavier. If someone tries this I would love to see it :).This would help eradicate the only flaw I've found. This being that when you grab a book and put it back after, the whole row moves slightly which results in more space between the books. Every time that happens you need to sandwich the row together again. Another fix would be to have some kind of anti-slip material under the book end that keeps it from sliding.