A comprehensive encyclopedia "covering the entire history of literature in the United Kingdom and the Republic of Ireland in the major literary languages (Anglo-Saxon, English, Welsh, Scots, Irish, and Latin)."
The Oxford English Dictionary, or OED as it's more popularly known, is a historical dictionary of English, covering the language from the earliest times to the present day. It aims to show not only the current meanings of words, but also to trace their development through time. Entries contain detailed etymological analysis, and are illustrated by quotations from a wide range of English language sources from around the world, making the OED a unique historical record. Available in print and in electronic version.