Let's say I'm a greek wealthy man from <greek city state A> and a philosopher that lives in <greek city state B> has just finished his book about something. How could I get my hands on a copy? Or how would I even know that that philosopher was writing something and finished it?
My question is not reduced only to the classic greek world, the same goes for middle ages Europe or similar contexts.
During the medieval period, religious houses (where most writing would have taken place) usually employed networks of couriers and messengers to disseminate texts they had produced. We can deduce this from the practice of obituary or mortuary rolls, which appear as early as the eighth century in Francia, and spread throughout Europe by the eleventh century. These rolls were produced, as the name suggests, upon the death of a prominent person, often the abbot of a monastery or abbey, but increasingly for members of the secular nobility as the format grew more widespread. The roll comprised an obituary of the deceased and prayers for their eternal soul, and would then be ferried around other institutions as both a notification and a book of remembrance, with members of other institutions encouraged to append their own remembrances and celebrations of the deceased, and note the intercessions they would be making on their behalf.
What is key to your question in particular is that in most cases, the roll would include a clause asking other institutions to provide hospitality and recompense to the messenger who brought the scroll, and to assist him on his further journey. Although it isn't specified if they are fellow clergy or a lay worker, they are often named personally in a style which implies they are a dedicated messenger rather than a monk doing the duty of carrying the roll.