I was re-organising my books after moving, and once again I noticed the frustration that is inconsistent text direction.
Since I'm sorting roughly by topic, I have books in different languages next to each other.
At the moment, I can say that Greek and French books are overwhelmingly bottom-to-top. English books on the other hand are running top-to-bottom.
I don't know how much of this is learned behaviour, but the English book spines are almost impossible to read without titling my head, while I can bottom-to-top without any conscious effort. Is it different for people raised in an English written culture?
Of course, CJ(K) readers can laugh at both of us, since their scripts can be literally written at any direction, without rotating the characters (my favourite script trivia is that Japanese for some time after modernisation was written horizontally, but as RTL, because they treated horizontal writing as vertical writing that was only one line tall).
That last one makes me wonder how RTL written cultures print book spines. It would probably make sense for them to use the English way.