Books were extremely rare during the Middle Ages. The few that existed were generally hand copies that monks or other writers made with much difficulty. Reading was truly reserved for the nobility as books were so limited.
The most common language for medieval works was Latin because the church was virtually the only educational institution. There were a few copies of works in Old English, German, and more but these languages were not consistent as writers wrote what they heard rather than follow grammatical rules.
During the Middle Ages, language was changed and improved. Capital letters were first employed and much of the grammatical syntax in use today first appeared.
In 1440, almost at the end of the Middle Ages, the German Johannes Gutenberg invented the first printing press in Europe. Within a few decades, books and papers that had previously taken months or years to write by hand were copied within hours. This is one of the reasons why during the Renaissance there was more literature compared to the Middle Ages.