I've read a lot of writing books this past year. Some I found useful, others . . . not so much. This is very subjective, though, because a lot of them contain the same basic information, just put in a different way. These are the ones that were compatible with my way of thinking, for some reason.
Wonderbook by Jeff VanderMeer is my absolute favourite. Even if you're a seasoned writer, you might like this book. It's gorgeous and very visual, full of inspiring images and imaginative writing exercises. It's also damn entertaining and well written; I had to read it from cover to cover before I could start any of the exercises. This book is just pure fun. I might post my attempt at the "Last Drink, Bird Head" exercise here later. That one's on the site, if you want to try it.
Chuck Wendig's writing books. Chuck Wendig is great. How many times do you find yourself laughing out loud while reading a writing book? Something might actually stick when learning is this fun. A lot of the stuff is available as blog post on his site, too, but it's easier to read these in e-book form, I think. A word of warning: if you have delicate sensibilities, these might note be the books for you. Maybe check out the site first to see if it's to your taste.
The Elements of Fiction Writing series. (The link goes to Amazon.) These are very useful. My favourites were Plot, Character and Viewpoint, Setting, and Description. The Description and Setting ones were the ones that really helped me understand writing; so many books tell you to avoid adverbs and adjectives, but that's not the whole truth. You need to find the specific, interesting adverbs/adjectives that help the reader see what's happening. The book on story structure I found very hard going. I think there're easier, if not better, books for that available.
Structuring Your Novel by W. M. Keiland is great for learning story structure. Most of the information in the book is available for free on her website. If you only check out the site, her posts on character arcs are great too.
So those are my favourites. What would you recommend?