Which one sounds natural? 1）Books are the ladders of human progress.

[NewHopeR]

If none, please make one.


1）Books are the ladders of human progress.
2) Books are stairs of human progress.
3)Books are steps toward human progress.
4） Books are humanity's progressive steps and ladders.
5) Books are steps and ladders by which humanity progresses.
6) Books are the ladder of the progress of mankind.

[SlickVic9000]

(not a teacher) I'd say 1 is the most apt statement of the group. I don't think I'd find myself quoting any of the others.

[JMurray]

None of these sound particularly natural to me, although #3 is probably closest.
Perhaps: "Books are the stepping stones of human progress".

not a teacher