What some have said about this book is true: it's more of a pep talk than a super informative instruction manual for editing. However, it's a pep that I think editors of all sorts need to hear. Even if you already know these things, it's nice to hear an authoritative source say: calm down, be flexible, don't drive yourself crazy. One of the big things that Carol Fisher Saller combats in The Subversive Copy Editor is the strongly prescriptivist tendencies of those who write and edit--style guides were made to help us, not enslave us. She gives great advice on when to stick strongly to accepted styles and when to let things go. She also lays out guidelines that will help you make these big decisions on your own. Another solid piece of advice she lays on thick is: put value on your own time and sanity. Her big three strategies when you've hit a tough task are 1) automate, 2) delegate, and 3) reevaluate. All good tactics.
She begins each chapter with examples of questions she's received during her time working on the Q&A section of the Chicago Manual of Style website. She ends each chapter with her sometimes serious, sometimes humorous answers.
Also, can I just say how cute this cover is? It's made to look just like a manuscript, from the printed-on rubber bands and paperclips to the yellow post-it on the front.
Anyway, I would consider this a must-read for anyone involved in editing, but especially for freelancers, since she spends a lot of time addressing freelancing-specific problems.