Someone on one of the forums I frequent mentioned doing an exercise from Ursula LeGuin's Steering the Craft, and I was sufficiently curious to put the book on hold at my local library. I'm glad I did, and I'll be purchasing my own copy in the near future.
It's emphatically a craft book, focusing more on the trees than the forest. Topics include sentence length and syntax, managing varying points of view, punctuation, verb tense, when to go into detail and when to skip ahead through summary, etc. But it's not a basic book by any means, more a master class on the finer points of the writing craft. I want to get my own copy and take time to work through the exercises because it's easy for me to think I know all that stuff already simply because grammar comes naturally to me and I've never had a problem with head-hopping. But there's always more to learn, and I don't want to stagnate as a writer, but to improve.