Just a note. If you want to build up your editing reference library, especially your home library, you might want to take a look at a Storify I did from a February ACES Twitter chat.
With more and more copy editors doing some sort of freelance work, building a good home reference library is important. But it doesn’t always take wads of money. Many of these references can be kept in bookmarks instead of book shelves.
Still, I’d recommend paying for a subscription or hard back to the stylebook you use most often.