Daniele Procida: "What nobody tells you about documentation (May 19, 2017) - A very good blogpost about different kind of manuals.
general writing tips:
"8 Tips for Better Readability" by Roby Collinge is a very good resource. However, some of the tips do not concern you as a writer. They are only relevant for theme development. As a writer, skip right down to tip 5.
Neil Patel: How to Write With Clarity: 9 Tips for Simplifying Your Message This is also not an article that is specific to documentation, but the tips are very good and general, especially for writing in clarity in mind.
Clarity means making your content easy to understand.
Strunk & White: "The Elements of Style is a classic. Many principles still apply.
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