For your reference library - Green's Dictionary of Slang
With Christmas approaching, wordsmiths might want to add the massive Green's Dictionary of Slang to their wish list.
At $360 and three volumes, this is not for the faint-hearted. But it gets high marks all around (so much so that it already has its own abbreviation, GDoS, according to World Wide Words).
(If you want something a bit more manageable, both in size and price, Cassell's Dictionary of Slang, also by Jonathon Green, is a good option.)