Everybody needs a refresher course in the basics, in what they know -- or think they know. Reading books by people who have been doing it longer than you is always helpful, and when it comes to writing, there's always something to learn, a new perspective through someone else's eyes.
Where Do You Get Your Ideas?
How Old is Your Reader?
Plotting a Great Story
What Editors Wish Writers Knew
Jan Fields' List of Five Good Tools that Can Get You Into Bad Trouble
Checklists. (especially helpful: Organization; Character and Settings; Sentence Structure; Mechanics)
Some of this information is presented as Q&A, so you get an idea of the questions other writers are asking too.
Very useful. No matter what you know or how much you think you know, it's always good to KEEP LEARNING.