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