Let's assume you're a young copy editor, just starting out. What books would you need to do your job? Thanks for asking. Recommendations follow:
1.) Dictionaries
More than one, please. Multiple dictionaries give you multiple opinions, quickly. Yes, it can be more fun to ask the person sitting next to you, but sometimes she's busy. I'm biased because I own dozens of the things, but no single volume (besides the AP style book, perhaps) will help you more.
2.) Some sort of thesaurus
I prefer The Synonym Finder, but any book that can jog your mind for extra words can help. Don't use it to produce unfamiliar words. Use it to stimulate your thinking.
3.) An almanac
This makes me sound old-fashioned, but having all the information on a printed page can help you. We too often turn to the Web when a paper source could do the job more authoritatively. You don't have to get sucked into a Google trap for every single question.
4.) A few usage books
Quite a few volumes about "correct" use of the English language crowd the shelves. Most won't help you directly. They make you think about the written word critically, though, and that's what copy editing is all about.