What is Reddit's opinion of The English Grammar Workbook for Adults: A Self-Study Guide to Improve Functional Writing?
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Have you considered picking up a book?

There's an excellent book called The English Grammar Workbook for Adults: A Self-Study Guide to Improve Functional Writing, highly recommended

I did a brief Amazon search and came across these two books, are they the kind of thing you're looking for? The English Grammar Workbook for Adults and The English Tenses Exercise Book - I'm not sure if there's anything specifically geared towards grammar for fiction writers but they might be a good place to start!