The Elements of Style: English usage reference book, from 1918. (Looking at this 2000 edition of Elements of Style, it's surprising to see some of the things I take for granted that were worthy of being put in a reference book as recently as 80 years ago: "Form the Possessive Singular of Nouns by Adding 's", "Use a Dash to Set Off an Abrupt Break", and "In a Series of Three or More Terms with a Single Conjunction, Use a Comma after Each Term except the Last" [although I disagree with the last, I learned it at school].)