Kenneth G. Wilson (1923–). The Columbia Guide to Standard American English. 1993.
AGENTIVE ENDINGS, -ER, -OR
The suffixes -er and -or are called agentive endings and usually are added to verbs to make new nouns meaning “someone who performs whatever action the verb stem describes” (driver; conductor). American English has many such words, for which the suffixes seem almost randomly chosen. In certain instances, however (see SAILER), each word preserves a semantic distinction; in still others the meaning is the same, but the spelling is in divided usage (See ADVISER). Other agentive endings added to other parts of speech include -ist (physicist) and -ian (grammarian). See also -ESS.
