This list of the best screenwriting books — all of which I’ve read — are useful depending on where you are in your screenwriting journey.
If you’ve already written something, I’d go for Akers’s “Your Screenplay Sucks!,” which will highlight the mistakes you didn’t know you made, and help you correct them. Also, Croasmun’s book on marketing your script is invaluable to help you navigate, with confidence, Hollywood’s world of “no.”
If you aspire to writing action, Martell’s “Secrets of Action Screenwriting” is a masterclass in suspenseful screenplay writing.
McKee’s “Story” is all about crafting strong characters, compelling scenes, and overall story structure. If you haven’t been able to attend his highly recommend seminars — attended by many Oscar- and Emmy-winning screenwriters — it’s all covered in this text.
A nice addendum to Mckee is Seger’s “Making a Good Script Great,” which will really help you improve your scenes.
And if comedy’s your thing, then Kaplan’s “Hidden Tools of Comedy” is the comedy reference book you need.
As you’ll find on Amazon, these screenwriting books are highly rated for a reason.
. Kaplan is a long-time comedy consultant in the entertainment industry who successfully has distilled his transferable knowledge into what is one of the best comedy books available. 
It’s easy as a screenwriter to tend to focus on dialogue, but the reality is stories need structure and conflict before they need any dialogue. The Doritos Super Bowl goat ad is a great example of why it is best first to focus on the story before worrying about the dialogue. 
Sometimes it’s tempting to start pounding out dialogue for a scene before you’ve fully plotted the story or thought about core character traits, but such hastily written dialogue quite often is the worst thing screenwriters write — at least during the first draft anyway — because the goal of the first draft is to finish a first draft, not to have in hand a refined industry-worthy screenplay. But it’s okay, because dialogue easily can be improved simply by