Oxford English Dictionary

There is nothing more fun than browsing through the OED. The books are huge and unwieldy, but the entries are exhaustive, listing every time the meaning of a word changes slightly in the literature, and quotes the lit. from which it came, mostly 19th century.

I got my copy used from a college library which was upgrading to a newer version. I love it and use it often. It also has many words you don't encounter these days.