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 resources include Merriam Websters Guide to Punctuation and Style.