Every serious editor (and writer) should have a Merriam-Webster's Dictionary and a copy of the most up-to-date Chicago Manual of Style on their bookshelf. The first one is an English dictionary to help you look up words and their correct usage and spelling, and the second is a style guide to help you cross-check your author's style of writing against industry standards. My other most trusted resource is the Merriam Websters Guide to Punctuation and Style. It covers the correct usage of punctuation and writing styles. Click on the pictures below to learn more about each guide, or for links on where to purchase your copies.
I wrote two of the guides (A Simple 5-Step Guide to Self-Editing and How to Work with Your Editor in Word Using Track Changes) to help readers in their writing.