The only paper dictionary worth having.
Except for the 'New Oxford Dictionary of English'.
Look for the 3-volume 1933 edition (+1975 supplement) available from most good used book stores for a song.
OED is also available online: http://www.oed.com/. It's the only dictionary you'll ever need. Unfortunately, it requires registration ($$), but many colleges and universities have a site-subscription for it.
Among the most renowned dictionaries of the English language. It is the largest and the most complete English dictionary ever compiled (20+ volumes)
It is based on historic principles. For many words, you have examples of usage dating back to many centuries.
See also MerriamWebsterDictionary for a free on-line resource!