Introduction
In the modern era, it is difficult to find someone who would be illiterate and uneducated, in fact nearly everyone is taught how to read and write at a very early age, but this was not the case in the Middle Ages, it was rather quite the opposite. Most people had insignificant or even no education whatsoever and did not know how to read or write, nor did they really need to, as most things were arranged through speech and they spent most of their days working to their occupation.
It could be reasonably assumed that half of The Nobility was illiterate, as their livelihood did not require them to be literate to be able to perform their duties. If a person of lower status wished to learn, The Clergy either did not allow them to or made them pay a price for education that they could not afford unless they were wealthy. This is believed to have been done in order to keep the reading of the liturgy to themselves to avoid potential readers from understanding different interpretations of what was written. The Church needed their priests and monks to be literate in order to understand the religious Christian texts and to also be able to read them out loud to the people, so they provided them with education.
Literacy
The definition of literacy during the Middle Ages could be interpreted in different forms, it could mean that a person knew how to read or write in Latin, had some form of education or knew more than one language. Latin was very important as it was the language of Christianity, and as such was also the language commonly used for important documents related to varying study disciplines and important texts.
Books
Books were written by scribes, who did their work in scriptoriums, commonly found in monasteries, but there were also freelance scribes who offered their services to those who could afford it and had the need to, usually being the nobility. Most scribes, however, spent their days copying books and manuscripts, often related to Christianity, in the scriptoriums of monasteries. It is assumed that many of these monastic scribes might not have even been literate or even knew how to read the texts that they were copying in the first place, but were simply copying mechanically, without reading or paying much attention to the content of the text, which is hinted at by the various small errors found in copying, most common of which was a lack of space between words where there should have been.
Education
After the fall of the Western Roman Empire and the preceding sacks of Rome and the destruction and pillaging of many other cities, libraries, and monasteries by barbarian invasions into Italy and other notable regions of the late Roman Empire, the Catholic Church took responsibility to preserve the remains of the Roman and Greek manuscripts that were rich in knowledge and to make many copies of them so that they might never be lost again. With this responsibility, The Church had become the main source of knowledge of arts such as Latin, Calligraphy, Logic, Rhetoric and in some places - subjects such as Arithmetic and other sciences, but The Church mainly avoided the subjects related to science and preferred to study theology and the arts aforementioned.
Education was very expensive, however, and as mentioned in Introduction most people, including nobles could do their duties and perform well without being literate. Due to the cost, only nobles and the wealthy were able to send their children to monastic schools. However, the preferred course of education for most nobles during the High Middle Ages was to either send their child to page and squire at another lord's court and take up the Path of Knighthood or to raise them in their own lands and holdings. In both cases, the child would have ideally been taught about obeying their superiors, understanding authority and courtly manners and behavior, along with trivialities such as - how to clean, maintain and wear armor, how to ride a horse, how to wield a sword and depending on the lord's court - to play board games, join hunts and learn how to read.