The book is nearly complete and just needs a final check to ensure that errors weren’t introduced during the design process and that there are no lingering grammatical errors.
Style guides and style sheets are both important documents to use when editing. Both style guides (standard and in-house) outline the style, grammar, and layout guidelines that the manuscript should follow. Editors should be familiar with both style guides and consistently apply them to the manuscript. Style sheets are just as important, and as an editor, you should always be sure that you match what is on the style sheet for the manuscript you are working on.