A lot of the elements of what we'd call "modernity" were either introduced or became widespread in the course of the first World War so I'm not trying to just be a jerk here; I think it really is a useful dividing line.
I'm not sure I agree to it being anything stronger than a 'catalyst' period, but regardless; as elsewhere, by 'modern' I meant 'more recent than it is'.
It's less than a decade from the cutoff date after which books stopped entering the public domain, so by Project Gutenberg standards, it is pretty modern.
Okay, fantastic, arguably a poor choice of word then. I think it's pretty obvious what I mean, and whether or not 1914 qualifies as 'modern' doesn't affect it in the slightest...
Not a poor choice of words at all, modern must be understood in context and you used it correctly. It would be impossible to ever use the word without a debate on definition otherwise. "How do I build a modern web app? .. Here, I found some relevant resources on vacuum tubes!"
And anyway, I confess I don't know the industry at all, but I doubt anyone would talk about 1914 as representing 'modern publishing'.