If you’re looking to assemble your own arsenal of writing resources, there’s no better place to start than with a dictionary or two and a style guide.
The first thing every writer needs is a good dictionary.
Choose one (or more, if you write for different audiences with their own lexicons) as your go-to for spelling and usage questions. Using a single(ish) source of truth will help you stay consistent, which keeps your editor happy. (And a happy editor is a key part of a happy life for a writer. Trust me.)
There are lots of dictionaries to choose from, from the American standby, Merriam-Webster to the venerable Oxford English Dictionary (known to word-nerds everywhere as the OED), and fortunately, many are available online for free.
The one I use for my HP fic is Lexico.com, which is powered by Oxford University Press (the folks behind the OED). It allows you to select US or UK English, and entries give American and British variants, which is useful if you, like me, can’t for the life of you remember whether Dumbledore has a gray or grey beard. The online Macmillan Dictionary has a similar function.
I tend to avoid online-only offerings like Google Dictionary and Dictionary.com because I’m old-skool and really only trust the folks for whom lexicography is life, but they’re likely perfectly fine for most purposes.
If you write fiction, I highly recommend creating your own style sheet that includes your general preferences plus anything that is specific to each work.
That way, you won’t have to waste time going back through 25 chapters of your Wolfstar epic to figure out if you capitalized Hippogriff.
I’d suggest starting by selecting a dictionary and a professional style guide to use as a basis for your style sheet, adding entries on the types of things you’re likely to need to look up (treatment of numbers, dashes, and so forth, for me), and tweaking the style sheet to meet your needs, noting where you differ from the dictionary and guide. Also note word spellings and treatments that are unique to your work. Some folks also put character and world-building notes here, but I find that makes the style sheet a bit crowded, so I keep them separate.
You can download a sample of the style sheet I made for my Epithalamium Series here.
I keep an updated list of suggested writing resources here.
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