The correct Latin plural is codices, although codexes is also often used as a plural form in English. The codex was an improvement over the scroll, because its pages and spine allow easier reading, and pages can be written on both sides. Though early codices were made with papyrus, medieval book makers used parchment or vellum for their pages, which made them very durable, but extremely expensive. The scholarly study of manuscripts from the point of view of book-making is called codicology.
The books of Pre-Columbian Mesoamerica had basically the same form, with long folded strips of paper (usually made from either wood bark or plant fibers, often with a layer of whitewash applied before writing), hence the ancient books of the Maya, Aztec, and Mixtec peoples, among others, are also known as codices.
A legal text or code of conduct is sometimes called a codex (for example, the Justinian Codex), since laws were recorded in large codices.