My brief, uncited, US-centric off-the-cuff answer is that the work generally started being spread around the globe in the 80s, and you started to see pro-level creators who had grown up with roots in multiple traditions in the 90s and 00s. Before then everyone pretty much grew up with exported US comics or local stuff.

Sadly I can't make any suggestions for other books on The History Of Comics, especially from a global view. You might find some interesting articles on the subject by browsing The Comics Journal. For Euro stuff, histories of Métal Hurlant (translated in the US as Heavy Metal) and 2000AD should give you a brief picture of where those influential magazines stood in their worlds before delving into the particulars of their own histories; I got no clue where to look for the history of Asian comics. (I know there has been some writing about the history of Métal Hurlant but I can't remember it offhand, feh.)

Somebody please write something better-cited and/or linked, this is just a quick attempt at something better than nothing.