Which material was popularized in ancient Egypt (and later used by Greece and the Roman Empire) for writing books and other documents — and was even discussed by Pliny the Elder, the first-century Roman author, in his 37-volume “Natural History” encyclopedia?

From the online edition of the Encyclopaedia Britannica:

Paper made from papyrus was the chief writing material in ancient Egypt, was adopted by the Greeks, and was used extensively in the Roman Empire. It was used not only for the production of books (in roll or scroll form) but also for correspondence and legal documents. Pliny the Elder gave an account of the manufacture of paper from papyrus.

And, from Book XIII, Chapter 21, of Pliny the Elder’s “Natural History”:

We have not as yet taken any notice of the marsh plants, nor yet of the shrubs that grow upon the banks of rivers: before quitting Egypt, however, we must make some mention of the nature of the papyrus, seeing that all the usages of civilized life depend in such a remarkable degree upon the employment of paper — at all events, the remembrance of past events.