There are hundreds of beginner-level writing books available for someone just starting out, but it's harder to find books that help with ongoing improvement after that stage. I have a few recommendations. The nifty thing is that these books all have a lot to offer for developing general creativity. Anyone from beginner through advanced in writing could get something out of reading these. They contain useful stuff for the rest of life as well, in my opinion.
The Talent Code, by Daniel Coyle. This is the most generally applicable book on my little list, because it's about the kinds of practice that lead to mastery, and what's happening in the brain as that mastery is growing. It covers deep practice, ignition, and master coaching, each of which are important in reaching high levels of success in creative work and sports, especially. My favorite thing about this book is that it gave me ideas about how learning to be an excellent writer is more like high achievement in soccer than it's like mastering the violin, because the former relies on learning to flexibly access a wide range of options, while the latter is about perfecting the one correct way to play any given note. Anyone interested in developing any kind of talent should read this!
Around the Writer's Block: Using Brain Science to Solve Writer's Resistance, by Rosanne Bane. THIS BOOK IS SO GREAT. I've increased my productivity so much since I started to follow the practices suggested in this one. The great thing is that, although it's a book specifically about writing, the information is EASILY transferable to pretty much any area of life. It's about recognizing the ways in which stress prevents creative thinking, and how to establish easy, helpful habits that will prevent the stress response from taking over and blocking you from doing what you want to do. The book explains how to establish methods of process, product, and self-care to keep yourself in the right state of mind for creative thinking. This gets my highest recommendation.
Making a Good Writer Great: A Creativity Workbook for Screenwriters, by Linda Seger. Okay, I know the title says it's for screenwriters, but really it's for everyone. Who doesn't need more creativity? This book includes chapters like "Pushing Your Mind to Another Creative Level," "Exploring Your Themes and Ideas," and "Mining the Riches from Your Dreams," as well as chapters more specifically dedicated to improving writing skills. The examples are about screenwriting, but any writer can benefit from them. I like to read books about screenwriting to learn from a different angle. They tend to give me a better appreciation of movies, too.
Wired for Story: The Writer's Guide to Using Brain Science to Hook Readers from the Very First Sentence, by Lisa Cron. Why do people enjoy stories? Our brains get interested in stories for specific, explicable reasons which are covered here. Learning what makes people curious, and what holds their attention, is useful for writing fiction. It's also crucial for giving good presentations, getting along with others, and being an interesting person. In a time when social interactions online and in person are more important than ever before, as people become increasingly adept and sophisticated in the social realm, this is valuable information.
2k to 10k: How to write faster, write better, and write more of what you love, by Rachel Aaron, has a really self-explanatory title. All right, this one is strictly about writing. It's especially good for planning and outlining novels, so that you know what you intend to write on any given day. That helps with getting started and allowing the words to flow faster.
I am always looking for good books to read, so I hope you'll comment with your own recommendations!