Microsoft Manual of Style for Technical Publications, 3rd ed. 2004.
Even if you’re not in software development or technical writing, this one would come in handy if you write the occasional “how-to” handout for staff or the public. It has recommendations for how to phrase instructions to minimize confusion, how to describe menus and buttons, etc. The second half of the book is comprised of a Usage Dictionary.
If your audience is multicultural, the Global Content section has interesting advice. For instance:
Art presents many globalization issues. [ ...] We especially careful to avoid images that are offensive in some cultures, such as holiday images and situations, whether work or social, involving men and women. Avoid hand signs; nearly every hand sign is offensive somewhere.
Contents: Documenting the user interface — Content formatting and layout — Global content — Content for software developers — Web content — Indexing and attributing — Tone and rhetoric — Accessible content — Common style problems — Grammatical elements — Punctuation — List of acronyms and abbreviations — Usage dictionary.