Deb Nemeth, my Carina Press editor, recommended “Goal, Motivation and Conflict. The Building Blocks of Good Fiction” by Debra Dixon. All I can say is, good karma to the two Debs 🙂
“Goal, Motivation and Conflict” gives you a framework to deal with your story’s conflict, making it coherent and tight.
On the other hand, the GMC framework isn’t rigid or prescriptive. When you stop to consider it, your characters do have internal and external goals, motivations and conflicts. Explicitly studying them and checking that they’re robust is just commonsense — sadly, commonsense sometimes takes a holiday in my life 😉
If you can grab a copy of GMC, I thoroughly recommend it. But don’t trust me, trust Deb!