The Chicago Manual of Style is the style manual used for most novels (as compared to AP, the style employed by journalists). An online version of it can be accessed at chicagomanualofstyle.com. A one-year subscription is $39, and also gives authors and editors access to a quick guide, a Q&A list, an online chat forum, video tutorials and a list of its own resources.
If you prefer a physical copy, the guide can be purchased in almost any bookstore. It can often be found in used bookstores for about $1, after giving writing students nightmares. This is an essential item to have at hand if you want to both your life and your editor’s to be a simple as possible. It will likely also save you a few dollars in editing fees.
The most current version of the Chicago Manual of Style is the 17th edition, which was published in September 2017.