Editors on Editing: What Writers Need to Know About What Editors Do
I highly recommend this book. I read it a few years ago; I'm re-reading it now. Editors on Editing is composed of articles from editors, authors, and agents. One gentleman used to write columns for Locus. :>
Facetious -- the report on Western Civilization's decline -- and serious, but always informative, Editors on Editing is a great book. For writers and editors.
Now that I have my own copy, I'm sure it will be filled with bookmarks.