The Writer's Handbook
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The University of Wisconsin–Madison Writing Center's online handbook, covering academic writing, citation styles, grammar and punctuation, and specific genres, so you can structure clear expository prose and document sources correctly.
More resources on Expository Writing
Writing in the Sciences
Stanford's online course on clear scientific prose, taught by Kristin Sainani. Covers cutting clutter, active voice, paragraph structure, the parts of a research manuscript, peer review, grant writing and publication ethics, so learners can draft and revise papers that read clearly.
Purdue OWL Expository Writing
Purdue Online Writing Lab's guide to the expository essay, explaining the thesis, evidence-based body paragraphs, transitions, and conclusion, so you can plan and write an essay that explains an idea or investigates a topic clearly.
writersdigest.com
WritersDigest.com is a comprehensive resource for writers, featuring craft articles, writing prompts, market guides, contests, and publishing tips to help aspiring and established authors improve their craft and get published.
Writing to Learn
William Zinsser argues that writing is a way of thinking in any discipline, using model passages from scientists, mathematicians, and artists to show how to explain complex subjects clearly, so you can write lucidly about technical material.