I'm sure by now that you have several writing reference books on your bookshelf. Some of my favorite are The Elements of Style and On Writing by Stephen King. From time to time I revisit these two books and find some tidbit or editing technique that I'd forgotten about.
Recently, a writer at a workshop I attended recommended Self Editing for Fiction Writers by Renni Browne and Dave King. I got a copy of the book and found it very helpful. Believe it or not, not all writers were English majors, and some people may have been snoozing in class the day the professor taught about the mechanics of writing good dialogue. If you happen to fall into this category of writing, you may want to check this book out, too.
It covers basic info like showing versus telling or creating character points of view, but I felt like the best parts were on creating proportion in your scenes. The sections on writing dialogue were interesting, too and gave me a few things to think about. The book does contain writing prompts at the end of each chapter and examples for you to read in order to test your critiquing chops, as well.
Even though your bookshelf may be weighted down with reference material already, I would recommend that you consider adding one more. Self Editing for Fiction Writers can't be any heavier than those Writer's Market books I know you have sitting there!