Three excellent books discussing plot, character, and the fundamentals of screenwriting. Recommended for novices to seasoned professionals to help them continue to cultivate their craft. 
The Art of Writing
Seven books dealing with the obstacles and struggles that all writers face. Great observations on habits, structure, story, and plot. The cliffnotes on how to "wrestle the bear" each and every morning. 
  1. The Elements of Style by William Strunk Jr. and E.B. White​​​

  2. On Writing by Stephen King

  3. The War of Art by Steven Pressfield

  4. Essay: ‘The Getaway Car by Ann Patchett (in her book, ‘This is the Story of a Happy Marriage’)

  5. The Art of Fiction by James Salter

  6. Plot & Structure: Techniques and Exercises for Crafting a Plot that Grips Readers from Start to Finish by James Scott Bell

  7. How to Write Best Selling Fiction by Dean Koontz

The Industry
It's a confusing industry filled with geniuses and hacks. These books will give you some insight on how the film industry works and what the aspiring writer will be up against. 
  1. Powerhouse: The Untold Story of Hollywood’s Creative Artist’s Agency by James Andrew Miller

  2. The Mailroom: Hollywood History from the Bottom Up by David Rensin

  3. Movie Speak: How to Talk Like You Belong on a Film Set by Tony Bill

Online Resources
Below are invaluable and necessary online sites to acquaint yourself with the Hollywood writing community. And also my friend Paavo Wilson's blog - when you need a breath of fresh air from the grind of pounding the pavement.