A style sheet can include preferred spellings, capitalization, number treatment, hyphenation, character or person names and recurring technical terms. For nonfiction, record how citations and references are handled. For a series, note details that need to remain consistent across books.
Keep the document concise and update it whenever a decision is made. Share it with the proofreader and production team so the same standards carry through layout. This small tool prevents unnecessary queries and makes the book feel carefully edited rather than assembled from separate pieces.