There are some previous discussions on the subject. But this is a starting point, not to discourage more questions, data, or debate.

A good summary is "Shift from reading out loud to reading silently". [deleted] started a thread citing articles, the summarizing, and later addressing a book by Saenger, commonly cited in these discussions.

Another brief reply addressing Saenger-like notions is /u/Astrogator at "Is It True That People Did Not read Silently (In Their Heads) Until The Dark Ages?".

Several discussions mentioned an anecdote of Saint Augustine concerning Saint Ambrose. /u/Tiako and /u/Bakuraptor discuss it in "Augustine found Ambrose's silent reading bizarre enough to comment on. Do we have more context for how reading used to be done and why this was so unusual?".