The Oxford English Dictionary.
It calls itself "the definitive record of the English Language," and rightly so. If you're a writer, a poet, or just a "word-lover" (first documented usage in 1857, says the OED), you might lose yourself in it for awhile. Possibly to the annoyance of those around you who don't quite share your enthusiasm. (Ahem.)
For those unfamiliar with the OED, here is its claim to fame:
The writer David Foster Wallace (1962 - 2008) once wrote:
We agree with part of that.
And anyway, no hacking necessary. Our libraries subscribe to the OED online so that you'll always have this classic resource at your fingertips — in the library, at home, or about town. Start here and select "Begin Using This Tool." Once you're on the OED website, select "Sign in" and enter your Jones Library card number.
Of course, if you'd prefer to flip through the physical, hold-it-in-your-hand, printed and bound OED, just visit our Reference Room. We've got 17 volumes worth on the shelf.
In closing, you're welcome.