Is it MLA that assumes everything is a book or journal article? Anyway, there is one widely used set of conventions that, although orientated towards the humanities, is absolutely hopeless not just for citing archival and manuscript material butalso pretty useless for supplying a meaningful yet not cumbersome form of citation to material from 1940s women's magazines I was using in a chapter of a volume for which it was the standard.

Also, I literally have seen (Plato, 2000) given as a cite.