A Style Guide--Any Style Guide
Consider your industry when you're style guide shopping. The Chicago Manual of Style is the most widely used style guide, especially with fiction and nonfiction books. If you write for Canadian publications you would probably go with the Canadian Press Stylebook. If you're a journalist who writes news-based articles, you probably often work in AP Style. Scientific writers often use APA Style, and if you are a literature- or humanities-based writer/editor, you would work with MLA Style.
What matters most is that you own at least one of these guides, because no matter how well you think you know your grammar and punctuation, I guarantee at some point in the midst of editing a hundred thousand–word book, you will forget whether a comma goes inside or outside of the quotation marks.
The Subversive Copy Editor
The Subversive Copy Editor is written by Chicago Manual of Style Online's Q&A editor, Carol Fisher Saller, and is therefore based around Chicago Style. An author's job is to tell a story. A copy editor's job is to correct an author's grammar, spelling, punctuation, and consistency errors. Saller teaches copy editors how to communicate with a difficult author firmly but tactfully, and how to compromise unnecessary changes (you don't always need an oxford comma), because ultimately both parties want the same end result: a great book (or article, or research paper).
The Oxford Dictionary of Synonyms and Antonyms
Synonyms and antonyms do to writing what paintings do to a room; they brighten up an otherwise boring space. A different painting can give a room a totally different vibe, just like a different word can completely change a sentence. Whether you're a writer trying to put your thoughts into words or an editor trying to more accurately illustrate a scene, a dictionary of synonyms and antonyms will become your best friend. You can always look it up on Google or the built-in thesaurus in your computer, but personally I like having a physical thesaurus that I know will remain correct and consistent.
Style: Lessons in Clarity and Grace
This book was required for my substantive editing class, and while there were some sections my teacher disagreed with or that we found to be a bit ambiguous, the overall lessons in this book are very useful when it comes to organizing writing. The title is pretty self-explanatory: the book discusses how to organize one's writing clearly and concisely.
Letting Go of the Words: Writing Web Content That Works
Most publications have gone digital. Many magazines have halted their print publications altogether in favour of online subscriptions, and on top of that, most of us have an attention span of about 8 seconds when it comes to reading web content. Author Ginny Redish teaches the reading how to organize web writing, how to use language effectively for the web, and even touches on SEO and social media writing.
Personally, I feel like the quality of most online publications has gone so downhill that web writers should be given a complimentary copy of each of these books just for the sake of preventing the dumbing down of the world, but until then, they're just my recommendations for writers and/or editors looking to put their highest-quality writing out there.