The Elements of Style by William Strunk, Jr. and E.B. White is a writing style guide which was first published in 1919 and was most often used as a grammar and usage guide by high-school English teachers.
It is a slim volume of rules for speaking and writing properly set down in list form with examples and, in the 2005 edition, lovely illustrations by Maira Kalman.
What I like about it is how relevant these same principles are for expressing individual style. For instance, Do not overstate, Avoid fancy words, Write in a way that comes naturally and my favorite, Omit needless words. In other words, edit, edit, edit.