Books as we know them (rectangular pieces of paper, attached along one edge, ready to be read by turning the pages) first appeared during the Roman Empire. In the ancient world, the standard had been the scroll, a continuous sheet of writing material that could be rolled and unrolled as one read.
(There used to be a hilarious video on YouTube, "medieval help desk," originating in Scandinavia, showing a monk upset because he couldn't figure out how to work a book, rather than a scroll.)
Medieval books were normally made of parchment. Parchment is sheep skin, carefully treated and bleached until it looks and feels like what we would call heavy paper. Parchment lasts very well across the centuries; I have seen and handled parchment over 1200 years old. As long as one's hands are clean, the oil in one's fingers is actually good for the parchment. Ink was made of a mix of oak gall and lamp black. For most of the Middle Ages, ink was more brown than black, though late medieval Italy was proud of its dark black ink.
All books were copied by hand, because the printing press was not invented until the fifteenth century, at the end of the Middle Ages. This meant that no two books were exactly alike. Because parchment is fairly thick, big books (like the Bible) were generally done in several volumes. Between the cost of the parchment and the slowness of copying, books were very valuable. Copying books was one of the works that monks undertook. A monastery with a very big library might have a hundred books. Books were regularly borrowed and handed around.
For most of the Middle Ages, most books were in Latin. A library might include various volumes of the Bible, many works of theology by the Church Fathers, philosophical works from classical antiquity, histories (Bede was quite popular), and collections of charters all copied into a single book. The latter was called a cartulary. Above is an image of a thirteenth-century cartulary.
Starting in the twelfth century, works of popular fiction, written in the vernacular (that is, Old French, Middle High German, and the like), might also be found in a library.
Paper was invented by the Arabs and first appeared in southern France in the thirteenth century. But it did not become common until the fourteenth century. Then, however, it was widely adopted. Medieval paper was "all rag content," much higher quality than modern wood-pulp paper. Late medieval paper is often in better condition than a twenty-year-old modern paperback.
Because paper was much cheaper and easier to produce than parchment, the price of books went down. Scribes also became sloppier, often writing in cursive rather than the carefully printed "book hands" of much of the Middle Ages.