Does the punishment fit the crime? Does the punchline fit the gag? Does the readership fit the effort of writing that book?
Ancient Roman authors were worried about book sales. Renaissance Europeans were, too. Medieval authors, however, were not that concerned with sales, but it doesn’t mean they didn’t write as much.
The book trade in Ancient Rome developed on the back of a market economy, strong infrastructure and high literacy levels. The Middle Ages did not boast any of these three features, at least not before the economic takeoff after 1150 AD. Then the rules changed, with new games and new players rushing in.
The book trade in the early and central Middle Ages was not driven by sales, but by needs. Books were required for ecclesiastical and political projects. They were cultural assets before they became economic ones. Paralyzed by a crippled economy, the medieval book trade increased literacy levels which in turn boosted the economy, which in turn helped the very same book trade grow, making the wheel turn ever faster.
Medieval authors were not worried about sales or about personal success like modern authors are. More often, they cared about the spread of ideas. For better or worse, seeking influence was what motivated many medieval authors. Influence in changing the culture or reinforcing orthodoxy.
Occasionally, however, medieval authors hoped their books would transcend the slender borders of their contemporary book culture and reach out on the other side of history far into the future. The English historian Henry of Huntingdon, writing in around 1135, deserves pride of place for his unparalleled sentiment. In the epilogue to his History of the English People, he hoped his book would still be read by the generations of 2135:
‘‘I who will already be dust by your time, have made mention of you in this
book, so that if, as my soul strongly desires, this my book comes into your
hands, you will, I beg you, pray to God’s incomprehensible mercy for me,
poor wretch. In the same way may those who walk with God in the fourth
and fifth millenia pray and petition for you, if that is mortal man survives
so long’ (H. Archdeacon of Huntingdon, Historia Anglorum, edited and translated Diana Greenway, Oxford 1996, Epilogue, c. 5)
He’s been lucky for the first 885 years, so his wish might come true.