Resource, reference, guide
The skills an editor develops over a lifetime of editing are the tools in her toolkit. Integral to developing and honing those skills are the books in her life. The books she reads for pleasure, making the rhythms of the language (or languages) intuitively her own. And the books she reads and studies specifically to learn more about how the language works, about how written (and visual) communication best informs and teaches, about how writing is parsed by readers, about the conventions and practices of the type of writing she will work with, and of course about those issues of mechanical style that make for uniformity and consistency on the page.
Here’s my current editor’s bookshelf (updated lightly in 2018), the collection of books I recommend for editors working with technical materials, narrative nonfiction, or fiction, in corporate environments, digital contexts, magazine editing, or book editing.