Paradoxically, most people find that documents written in the early part of the Middle Ages (roughly 500-1000 CE) are easier to read with an adequate knowledge of the language than later texts leading up to the invention of the printing press. The reason is that Europeans had continued the writing styles of the Roman period but this gradually progressed into an elaborate system of shorthand and compact letters as monasteries developed their craft.
The most significant writing styles of the early medieval period include Uncial Script, Insular Script, and Caroline Script. There are many subsets of these types, commonly with the term "minuscule" for compact writing in certain documents or manuscripts. These styles are characterized by gently curving, uniform letters, many of which are easily legible to untrained modern readers. Insular Script takes its name from its evolution and popular usage in Ireland and England. Caroline Script was quite universal in Continental Europe, hence being named after the Carolingian Dynasty. Although it is more elaborately stylized than most texts, the Book of Kells is an example of Insular Script.
As writing techniques evolved, monks became extremely proficient at stuffing the greatest amount of words onto a page as possible. The reason is that while paper and sometimes papyrus were in use, books were written on very expensive animal skin parchment (often called "vellum") so that they would last for centuries. The result is that individual letters developed much more concise sharp angles, relying on as many straight vertical lines as possible. In many cases, especially in "pocket bibles", the writing is amazingly tiny and a magnifying lens may be necessary to distinguish the letters.
These styles were pejoratively named "Gothic" because later Italians regarded them as barbaric, although the term Blackletter is frequently used to describe them. Blackletter later became more uniform by being incorporated into print it remained popular in German-speaking regions until World War II. Other areas developed their printing fonts from pre-Gothic writing styles, and handwriting moved back towards the early medieval and Roman period styles.
Many concise documents were written with an elaborate system of shorthand. Shorthand of this period took the form of abbreviating entire words with a unique character or by representing letters or common prefixes and suffixes with lines over the word. For example, "M" or "N" at the end of a word was often written as a line over the final vowel, and the word "et" had several symbols to represent it. Another example is that the word "Christ" was denoted by "XP" in reference to the original Greek spelling.
Proficiency in reading minuscule Gothic hands requires an excellent knowledge of Latin and a great deal of practice to become familiar with the hundreds of shorthand marks that were commonly used.
Owning any book in the Middle Ages was outlandishly expensive because of the labor involved in creating them, so most were of a relatively plain style that included lightly decorated initials to denote the beginning of a new chapter or section. Books owned by wealthy individuals, however, were often lavishly decorated. Highly decorated manuscripts are said to be "illuminated." Capital letters often included drawings within and around them, often depicting the subject of the writing. Other illustrations were merely artistic flourishes, and often you can see animals or mythical creatures, called "grotesques", decorating the page. Illumination could entail gold leaf or expensive pigments made from precious materials like lapis lazuli.
Majestic examples of illuminated manuscripts include the Book of Kells, the Maciejowski Bible, and the Book of Hours of the Duc de Berry.