I had some trouble hunting this down, but it has been discussed on Hacker News before, and I really loved it.
A book where you can enter "sport" and end up with "a diversion of the field" --- this is in fact the opposite of what I'd known a dictionary to be. This is a book that transmutes plain words into language that's finer and more vivid and sometimes more rare. No wonder McPhee wrote with it by his side. No wonder he looked up words he knew, versus words he didn't, in a ratio of "at least ninety-nine to one."
There are excellent directions at the end of the post for setting it up on your Mac. I completely recommend it.