Why do we say that British constitution is unwritten, although we read it in books?

The expression describing the Constitution of the United Kingdom as "unwritten" is based on the concept that the set of laws and principles under which the United Kingdom is governed is the Constitution. Unlike many other nations, the UK does not have a single constitutional document. This is sometimes expressed by describing it as uncodified or "unwritten" constitution, although much of the Constitution is embodied in written documents, statutes, court judgments and treaties. The Constitution has other sources, including parliamentary constitutional conventions, royal prerogatives and historical tradition.