Off the Shelf
Writing advice books are like diet books. They all say "here's this thing I tried and it worked for me, so it must work for you too." Wouldn't it be nice if that were the case? But, the truth is, just like how-to-lose weight, all you can really get from how-to-write books is "Here's this thing that worked for me. You're welcome to try it. No guarantees." That's fine. That's fair. As long as you're trying, you are bound to find something that works.
I've only read a small handful of instructional books. There are a few more sitting on my shelf, but I've been deterred from going too in depth, because while some books are helpful and inspiring (Hello, Stephen King) many of them are just collections of "you can do it too" trope. There is, however, one thing that every book, every guest speaker, every fellow writer seems to agree on; in order to be a writer you have to write. Don't just think about it. Don't just dream about it. Definitely, don't just read about it. Put your butt in the chair, your fingers to the keyboard and write. Sounds like pretty solid advice to me, but I think there is still some validity in reading the occasional instructional manual. Even if all you get from it is a renewal of inspiration, then you've done something good with your time.
If you're taking a break from the keyboard or if you're just looking for something to help you refocus, here's a list of a few instructionals that I've really enjoyed. If you have any to add, please chime in.
On Writing: A Memoir of the Craft by Stephen King Writing the Blockbuster Novel by Albert Zuckerman The Elements of Style by William Strunk Jr. & E.B. White How to Write What You Want & Sell What You Write by Skip Press