Kenneth G. Wilson (1923–). The Columbia Guide to Standard American English. 1993.
espresso, expresso (n.)
Espresso (pronounced es-PRES-so) is the beverage produced by forcing live steam through finely ground dark-roasted coffee beans. The variant expresso is frequently considered Nonstandard—it is both a mistaken spelling and a mistaken pronunciation (ek-SPRES-so)—but some dictionaries now accept both pronunciation and spelling as Standard. Be aware, though, that some conservative users of Standard English accept only espresso (plural: espressos). See PLURALS OF NOUNS ENDING IN -O.
