Books and Manuals 
The Elements of Style, Fourth Edition
by William Strunk, Jr. and E.B. White
A Writer's Reference
by Diana Hacker
Say What You Mean! A Troubleshooter's Guide To English Style & Usage
by R.L. Trask
Word Court
by Barbara Wallraff
Oxford Dictionary of Idioms
Edited by Jennifer Speake
Webster's New World Dictionary
of American English
Eats, Shoots & Leaves:
The Zero Tolerance Approach to Punctuation
by Lynne Truss