One of the things that I always impress upon other editors, is to build your own 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. In fact, sometimes that is more difficult when you work on a wide variety of documents that use the different Style Guides. Having your own copy can help you find answers quickly and effectively.
There are also many reference books out there that can really help make you the best editor you can be, including grammar books, foreign language quick guides, the best dictionaries and thesauruses, a few How to edit guides, and so much more.
There are also website that can be handy for that quick reference, or double checking a citation style.
Here you can find what I consider the most commonly used Style Guides as well as some of the more obscure (lesser used) that I have come across in the past decade or so.
Please Note: These are the the guides used predominately here in the United States as that is where I live, and a couple of guides from other countries that I find myself using as well.
I have provided the links to the official websites for these as well as links to them on Amazon (or the PDF if available).
Most Common Style Guides
Books mentioned on this page on Amazon: