Everywhere I've gone this summer, people have recommended Save the Cat! (Blake Snyder, 2005) to aspiring novelists. It took 3-4 mentions to get through to me, because it's a screenwriting book, and I figured there were advice books just as good that were actually designed for novelists.
I'm glad I decided to give it a try. It's a clear, chatty description of how to structure an exciting, marketable story--sort of a simplified Hero's Journey. Since I'm a big fan of Christopher Vogler (who, now that I think about it, is also primarily advising screenwriters), it made a lot of sense to me. I'm also just at the right point for such a book, since I've finished a rough draft and am about to plunge into revision mode. Since I'm planning some big structural changes, I think I'm going to try using his storyboard and beat techniques. Not slavishly, but as a loose framework. That's been the lesson of my WIP--that I can't get by without a framework in a tightly plotted action story that's designed to be first in a series the way I could when I was writing standalone romances.
Snyder is an unabashedly commercial writer, to the point where art film/litfic types might not find his advice relevant or helpful. Since I'm trying to figure out how to make my book simultaneously accessible and intelligent, I like his approach as a guide for how to make my history geekery fun for people who haven't spent as much time immersed in military histories and Wellington biographies as I have.