American lexicographer Joseph Persico spent over a decade in Buenos Aires, where he decided to write the first porteño-English dictionary with over 2,400 entries. His works focuses on those regionalisms that can be found in the area of the City of Buenos Aires, and just 60% of the entries are also found in common English-Spanish dictionaries. Persico's dictionary is really one of a kind and you cannot not have it if you are a native English speaker with an interest for castellano rioplatense and lunfardo.
The Persico's Lexical Companion to Argentine Spanish is available for sale both in Argentina and in the US in the US though Amazon Amazon for just 17.99USD. You can get updated information on where to find it in Buenos Aires on the book's facebook page.
Perfil wrote an article on the book, available here.