The latest publication of the Oxford English Dictionary (2nd Edition) contains definitions for about 600,000 words. I don't know how many volumes it runs to, but that'll be one of your best bets. I think I'm right in thinking that it's available digitally.

If anyone is interested, the 3rd edition is currently being worked on, and is estimated to be completed by 2037, and has an average of 4000 new words a year added to it.

http://en.wikipedia.org/wiki/Oxford_...#Third_Edition