The Elements of Style, by William Strunk and E.B.White, ISBN 0-02-418200-1
The perfect "little book." Read it first. An online version is available at  http://www.bartleby.com/141/strunk.html

Style, by Joseph Williams, ISBN 0-673-18058-1
Read this book next.

Simple & Direct, by Jacques Barzun, ISBN 0-226-03868-8
The master’s classic.