Aesthetically

Organizing your books aesthetically: books that look beautiful together - spines with similar patterns or designs or illustrations or fonts, &c. - stand together.

Alternatively: use the spines' designs to build patterns. You can build stylized pictures or maybe write messages or draw elder signs. One version that was suggested to me was to arrange my books so the shelf would look like the old Swedish tv test image:
Instead, I tried to arrange them into a tapestry of a forest scene.
It didn't work.