Take a look:

American (US) English. Invaluable if you're self-editing, this is the quintessential reference work for authors, editors, proofreaders, indexers, copywriters, designers, and publishers. I recommend you consider buying an online subscription. It's a little more expensive, but much easier to navigate.

Now in its 17th edition, The Chicago Manual of Style Online is a mandatory reference work with more than two thousand hyperlinked paragraphs. I feel the annual subscription is worth while, even though it's slightly more expensive, because it's quicker and easier to navigate than the print version.

Available in print and ebook, Merriam-Webster's Collegiate Dictionary is the preferred dictionary of The Chicago Manual of Style. It is now in its 11th edition.