I am deep into revisions for my YA novel, and it is undergoing a major rewrite right now. I am taking out a main character, which means that the last 100 pages have to be completely rewritten. Why am I doing this? Because I am the queen of tangents and I think this plot twist I added at the end may actually be a monster-type plot tangent that took on a life of it's own and overtook the last part of my book. I love it, but I am doing an experiment to see what would happen to the story if I took him out. We'll see. I did it with Meg's Melody, and I liked the result, so that's the version I turned into CFI, and we know how that turned out.
So while I'm doing this revision, I decided to pull out my trusty writing book, Plot and Structure by James Scott Bell. I love this book. It is seriously a must have. I borrowed it from a friend in my critique group about a year and a half ago when I was doing revisions for Meg's Melody and liked it so much that I had to get a copy for myself. Right now I'm evaluating my character arcs for my three main characters of my YA novel (which is undergoing a title revision, as well.)
I also love Ann Lamott's Bird by Bird and Orson Card's Character and Viewpoint. What other good writing books are must haves?