Be careful when you choose a dictionary: small paperbacks generally aren't complete enough for business writers, and many hardback dictionaries are out of date or badly edited. This is my favorite.

Using a thesaurus is risky. It can help you find a word you know but have forgotten. However, it can hurt you by suggesting a word that isn't appropriate or effective. This is one of my two favorites.

Usage guides cover the etiquette of language, from when you can split infinitives to whether you can say "prioritize." Of the many usage guides available, I recommend this one.

This book, with an obvious nod to Strunk and White (which it surpasses), lists 67 principles of good writing, with about two pages each of details and examples.

Although it doesn't focus on business writing, Williams's book is another excellent resource for revision.

This book sums up what I've learned from nearly 40 years as a business writer.