Style Guides

One of the things that one should always impress upon other editors is to build a collection of Style Guides and a Reference Library. No one can keep all the information for all the styles of editing out there in their head. Sometimes that is more difficult when working on various documents that use the different Style Guides. Having a copy can assist an editor with finding answers quickly and effectively.
Many reference books can help any editor be the best editor they can be, including grammar books, foreign language quick guides, the best dictionaries and thesauruses, a few How to edit guides, and more.
Some websites can be handy for that quick reference or double-checking a citation style.
Here are the most commonly used Style Guides and some of the more obscure (lesser-used) that I have come across in the past decade or so.
-Galen
Please Note: These are the guides used predominately here in the United States as that is where Galen lives, and a couple of guides from other countries that she uses frequently.
Links to the official websites for these are provided.

Most Common Style Guides

American Psychological Association (APA)
Associated Press Style (AP)
Chicago Manual of Style (Chicago)
Modern Language Association (MLA)
University of Oxford Style Guide (Oxford) – PDF Direct Link

Books mentioned on this page on Amazon: