Chicago Manual of Style

The Chicago Manual of Style is probably the most relied-upon of all the style manuals, and is very comprehensive. It’s hefty, but worth picking up when you’re having trouble. 

It’s not really as scary as it looks. You can get an idea of the common-sense approach the Chicago people take by checking out their Q&A page.

Sure, there are more user-friendly options for writers, but for the professional editor, The Chicago Manual is indispensable. I use it to jog my memory, to solve sticky problems (like how to hyphenate “early twenty-first-century French furniture”) and to help with style decisions, along with The Associated Press Stylebook and others.