Origins
The word 'library' enters English around 1374 from Anglo-French 'librarie,' from Latin 'librārium,' meaning 'a chest or case for books.' The Latin noun derives from 'liber,' which in classical Latin meant 'book' but originally — and crucially — meant 'the inner bark of a tree.' This etymological fact connects the most exalted institution of human knowledge to one of the humblest materials in nature: the thin, pale layer of bark that lies between the outer rind and the wood of a tree trunk.
The connection is not metaphorical but historical. Before papyrus became available through trade with Egypt, the early Romans and other Italic peoples wrote on strips of inner bark, called 'liber.' The bark of the linden tree (Latin 'tilia') was particularly favored for this purpose. When papyrus and later parchment replaced bark as writing surfaces, the word 'liber' transferred from the material to the object — from 'a strip of bark you write on' to 'a thing that is written on' to 'a book.' The PIE root behind 'liber' is *lewbʰ- (to peel, to strip off), referring to the act of stripping bark from a tree.
This material etymology has a parallel in the word 'book' itself, which derives from Proto-Germanic *bōkō, related to 'beech' — the beech tree, whose smooth bark was used by Germanic peoples as a writing surface. Thus both 'library' (from bark) and 'book' (from beech) preserve memories of a time when trees were the first medium of literacy.
Latin Roots
The Latin 'liber' produced a small but important family of English borrowings. 'Libretto' (the text of an opera) is the Italian diminutive, literally 'a little book.' 'Libel,' now meaning a defamatory publication, originally meant simply 'a small book' or 'a written declaration' (from Latin 'libellus,' diminutive of 'liber'). The word 'librarian' (one who tends a library) entered English in the seventeenth century.
An intriguing false cognate deserves mention. Latin had two words spelled 'liber': one meaning 'book' (from *lewbʰ-, to peel) and another meaning 'free' (from *h₁lewdʰ-, relating to people, to freedom). These are different words from different roots that happen to look identical in Latin. 'Liberty,' 'liberal,' and 'liberate' come from 'liber' meaning 'free'; 'library,' 'libretto,' and 'libel' come from 'liber' meaning 'book.' The coincidence has inspired many poetic reflections on the connection between books and freedom, but etymologically, the two 'libers' are unrelated.
The semantic history of 'library' across European languages reveals interesting divergences. In English, 'library' means a collection of books (especially one available for public or institutional use). In modern French, however, 'librairie' has shifted to mean 'bookshop,' while the institution English calls a 'library' is called a 'bibliothèque' — from Greek 'bibliothēkē' (biblio- + thēkē, 'a place for books'). Spanish 'librería' can mean either bookshop or library, while 'biblioteca' (also from Greek) specifically means library. This is a rare case where English preserved the Latin-derived word for the institution while Romance languages replaced it with the Greek-derived term.
Modern Legacy
The physical library — a dedicated building housing a large collection of texts — has ancient roots. The library of Ashurbanipal at Nineveh (seventh century BCE) contained over 30,000 clay tablets. The Library of Alexandria (third century BCE) was the most famous of the ancient world, aspiring to contain all human knowledge. The Roman 'librārium' could be private (a wealthy citizen's book collection) or public (Rome had at least 28 public libraries by the fourth century CE). The medieval monastery library preserved classical learning through the centuries when no other institution could. In each era, the word 'library' — or its ancestor — named the place where knowledge was gathered and protected from time.