Steering the Craft: Exercises and Discussions on Story Writing for the Lone Navigator or the Mutinous Crew by Ursula K. Le Guin is a short and fascinating book on the craft of writing. It ranges from the basics of writing (punctuation, grammar, sentence length, adjectives and adverbs, verb tenses) to higher levels of craft such as point of view, author and character voice, narration, use of repetition. One section that surprised me was on how your writing sounds when read aloud; it hadn’t occurred to me that this might be important.
There are ten writing exercises, one per chapter (some with multiple parts) that give you practice in applying the lessons of that chapter. Some of them look quite challenging for me. I haven’t tried them yet, but plan to do so as breaks for my current Work In Progress.