Around the 11th century books that were written as we know them today were starting to be implemented, before that books that were written in scriptio continua, this is, with total lack of spaces and punctuation marks.

This, today, still prevails in the shape of email addresses and websites.

Before punctuation and spaces made appearance in texts, there was a need of a "reader" able to rather guess where the pauses and spaces were at so that people may understand them. This included expressing gestures, changing the tone of voice along with its volume.

When the need for a reader was not needed a new custom was born: Reading silently. Before that, reading was tightly bound to the vocalization of the text itself. Augustine of Hippo mentions in his "Confessions" that:

When Ambrose read, his eyes ran over the columns of writing and his heart searched out the meaning, but his voice and his tongue were at rest... I have seen him reading silently, never in fact otherwise ... I asked myself why he read in this way.

This is mentioned as a rarity, when reading out loud was the normal thing to do.

The scriptio continua hindered the discern of separations and meaning, reading out loud helped on this. To read, oratory skills were needed and it was expected from the reader to place the pauses where they should be at. There were many personalized styles of reading!
As silent reading gained popularity, study changed. During ancient times in the middle ages, a monk read while the rest listened. Of course, books were expensive and having one for each had a prohibitive cost if possible.
This, evolved into another custom: Paris, 1259, in the Couvent des Jacobins de la rue Saint-Honoré it was asked for the students to bring a copy of the text that was being studied. This way, as one read, the rest followed by reading themselves. The Monks claimed that this way the students understanding could be better. Of course, books were expensive. To solve this, the professors organized sessions of copying, cheaper copies appeared. Other people rented the books from private collections but at a high price, besides the monetary payment the owners asked for a copy to be added as part of it.

Starting from the 13th century -thanks to silent reading- studying became a private thing. The reader was fully immerse in the text. Not only reading became private, writing also became private, THINKING became private! Reading silently made reading "forbidden things" possible.
In ancient libraries, the books were chained, you had to read standing. In them silence was not obligatory, the only protection against other reader's voices was... your own voice.
As silent reading was becoming a standard, libraries became the silent places we know today.

The transition to silent reading had some nefarious consequences. The church was very afraid of this (losing control), silent reading was frowned upon by them, they did not like not knowing what people were reading. Silent writing was also implemented; before silence prevailed text and speech were one. A surveillance over the content of the books started, in 1346 decree claimed that every book not approved at the Bibliothèque de la Sorbonne should be burned.

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A new mental process was born, abstract reading, where concepts and ideas were adopted instantly by the reader without verbal words in between.

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Sources:
1, 2