Oh dear.

This was way more clever than it seemed at first glance. 😂

Several of my bookshelves are devoted to a single (Italian) publisher, namely Adelphi. They have a strong esthetic coherence (Google them and you will see what I mean) and within that publisher I sort by series and then by number.

Otherwise I sort by genre/category (e.g. Tolkien, scifi, photography, fiction, history,...) And within it alphabetically (by author and then by title). When I have some other cases like Adelphi (e.g. Sellerio with its blue books or Penguin classics paperbacks) I strive to achieve adjacency.

Any other approach is clearly madness and nonsense! (I move books around every few weeks :-P )