Merriam-Webster's Collegiate Dictionary

This is the dictionary required by publishers I've worked with. There is an online edition of M-W's huge unabridged dictionary.

The Chicago Manual of Style —16th edition:

This is practically the bible for copy editors, required by publishers, and often the final word on questions we have about style.

There is an online version that has great search functions you can use to quickly to find the relevant grammar/punctuation rule(s). You can get a free 30 —day trail.

Style: The Basics of Clarity and Grace

by Joseph M. Williams. Practically a graduate course in crafting good sentences.

The Careful Writer: A Modern Guide to English Usage

by Theodore M. Bernstein

Watch Your Language

by Theodore M. Bernstein

Words into Type

from Prentice-Hall

The Well-Tempered Sentence:

A Punctuation Handbook for the Innocent, the Eager, and the Doomed by Karen Elizabeth Gordon.

The Transitive Vampire:

A Handbook of Grammar for the Innocent, the Eager, and the Doomed by Karen Elizabeth Gordon.

Sin and Syntax:

How to Craft Wickedly Effective Prose, by Constance Hale

Spunk and Bite:

A Writer's Guide to Punchier, More Engaging Language & Style, by Arthur Plotnick

Eats, Shoots & Leaves:

The Zero Tolerance Approach to Punctuation, by Lynn Truss