One of my favorite books on writing is Techniques of the Selling Writer by Dwight Swain. As most writing teachers will tell you, this is the bible for writing commercial fiction. It’s a valuable resource you’ll use throughout your writing career. Just a word of warning, though. It’s very dense, and you might not get it all the first time through. Don’t worry about it. You’ll get the parts you’re ready for as a writer. Then I promise you’ll pull it out again and again during your writing career – maybe just to read sections — and then you’ll say, “Oh, I get it.”
After over twenty years of writing, I still use it today and am still finding new information that I didn’t ‘see’ before.