What is said regarding the hand to hand exchange (of books of knowledge), and the writing of knowledge by religious scholars to different countries

Narrated Anas bin Malik: Once the Prophet (sallallahu 'alaihi wa sallam) wrote a letter or had an idea of writing a letter. The Prophet (sallallahu 'alaihi wa sallam) was told that they (rulers) would not read letters unless they were sealed. So the Prophet (sallallahu 'alaihi wa sallam) got a silver ring made with "Muhammad Allah's Messenger (sallallahu 'alaihi wa sallam)" engraved on it. As if I were just observing its white glitter in the hand of the Prophet.