Here in the States, if you are writing fiction and nonfiction alike for readers here, then The Chicago Manual of Style, Sixteenth Edition (16CMS), is the preferred grammar and reference guide, while Merriam-Webster's Collegiate Dictionary, Eleventh Edition (Web11), is the preferred US spelling reference.
However, if you are publishing to newspapers, the Associated Press Stylebook (AP) is preferred for that medium—not to be confused with the APA style used for the American Psychological Association. There is also the MLA Handbook for Writers of Research Papers, which is self-explanatory, and the Christian Writer's Manual of Style based on these particular subject matters and intended audiences. Just naming these specialized alternatives shows you the pool of reference materials you have to choose from.
So pick one. Use it. Refer to it. Study it. Confirm, and defend, your style choices made.
If your home base is not America, then your country of residence has its own particular guides. Or even if you are within the United States but want one edition of your book specifically targeting your readers in Britain, then accommodate that readership by embracing the rules of that land. Hence for England, I believe the Oxford English Dictionary (OED) seems to be the spelling reference book while the companion New Oxford Style Manual appears to be the UK's grammar guide.
Best wishes all!
Denise Barker, author + blogger + copy editor