Associated Press Stylebook and Briefing on Media Law
While directed primarily at journalists, if your job includes any formal writing this should be on your bookshelf.
Sloppy writing reflects poorly on you and your organization. The Associated Press Stylebook can help you avoid many common mistakes. For example:
Finding what you need is easy. The book is organized alphabetically by topic. If you want to know when to capitalize a job title, look up "capitalization." To find out the proper use of "who" and "whom," look up "who." The hard part is knowing what you don't know. Often we don't have any idea that our lifelong writing habits are wrong.
Outside of the main body there are additional sections that can be very helpful:
The only other caveat is that it is intended for an American audience. If you are writing for another English-speaking audience, there are a few -- a very few -- instances when The Associated Press Stylebook spelling and usage guidelines are incorrect.