Whenever I teach Common Problems of Beginning Writers at the Pikes Peak Writers Conference, I tell the audience that if you can only afford one book on writing, Christopher Vogler's The Writers Journey is the book to buy.
The hero's journey is a great structure to compare your plot to and thereby understand why you may have plot problems. His character archetypes help you better understand the functions your various characters are performing.
Jimmie H. Butler
Founder, Pikes Peak Writers Conference
Author of The Iskra Incident, Red Lightning-Black Thunder, and A Certain Brotherhood