The Chicago Manual of Style: The Essential Guide for Writers, Editors, and Publishers (14th Edition)

Rating:
Summary: a spuriously reasoned defense of bad usage
Review: Whatever advice you find here that is different from advice you might find in a conventional grammar book is mostly advice you should not follow. Get hold of a conventional grammar book, say, "The Elements of Grammar". Get hold of a good dictionary, an American Heritage first edition (if you can find it). Get hold of Strunk and White's "The Element of Style". Get hold of the second edition of Henry Fowler's "Modern English Usage" (edited by Ernest Gowers). You're set.

Rating:
Summary: Excellent reference for all writing and publishing
Review: Whether fiction, essays, or technical writing, you should use this book as your primary guide to style, punctuation, and general writing. How should you punctuate an abbreviation? How do you capitalize a word? CMoS will tell you.

Rating:
Summary: Buy this.
Review: You should buy this book. Sure, it's got shortcomings and even some outright errors (e.g. capitalizaiton rule for "French republic" with a lower-case "r"!), but this is the most reliable, comprehensive style guide out there. Supplement a style guide, as always, with your personal list of exceptions--but defer to "Chicago" wherever you can. You'll be glad.