Kenneth G. Wilson (1923–). The Columbia Guide to Standard American English. 1993.
casket, coffin (nn.)
Coffin is the generic term in English, but casket has been an American euphemism for coffin for more than a century, credited usually to undertakers’ efforts to prettify death. There is frequent objection to casket, but it seems well established, with some evidence of a semantic distinction: a coffin (the regular British term) is usually coffin-shaped, tapered roughly like a flat-faceted mummy case; a casket is essentially a regular rectangular box. (People used to keep jewels and other valuables in small caskets, but today these are jewel boxes, perhaps in part because casket has become so funereal.)
