I would not purport to speak for Grant Barrett, but I believe he might recommend the Official Dictionary of Unofficial English or the more genre-specific The Oxford Dictionary of American Political Slang (both edited and compiled by Grant Barrett). I believe both he and Martha Barnett have recommended the Historical Dictionary of American Slang, but I might have the wrong title for that one. I'm sure someone here will correct me.

That said, slang dictionaries would seem to be much more difficult to compile than standard-English dictionaries. "Standard" American or British English transforms slowly. But, in our increasingly-connected age, new slang arises, catches afire, and burns out in the span of months, not years, and everyone is on to new slang in a similar cycle. Many of the words with any staying power have to do with the technology that itself fosters this rapid transition — e.g., "google" or "friend" as verbs. My point is that any slang dictionary that is not some kind of online wiki-type collection is going to be outdated before it ever goes to print.

Historical and etymological dictionaries are still a lot of fun, though (one of my favorites, though it is very dated and very British is The Facts on File Encyclopedia of Word and Phrase Origins (3d ed., Robert Hendrickson ed. 2004). It describes the origins of some interesting phrases and words, and makes for good random reading.