In which I reveal the greatest book on screenwriting ever published, the greatest book on screenwriting never published, and a couple of runners-up. But first, a history lesson.
Syd Field essentially created the three-act structure for screenplays, and most books on screenwriting from the 80s and 90s were devoted to holding up or rejecting “the paradigm,” as he called it. 
The paradigm had three beats: the inciting incident, which kicks off the action, landing somewhere early in act one; plot point one (PP1), the twist that sets the stage for act two; and plot point two (PP2), the further twist that propels the action into act three. So, for example, if you were writing Jaws, the inciting incident would be the discovery of the first victim, PP1 would be the decision to hire Quint to hunt the shark, and PP2 would be the moment when the shark turns the tables and begins to hunt them.
With a page of script roughly equal to a minute of screen time, a two-hour movie could then be divided into Act One (the first 30 pages), Act Two (the next 60) and Act Three (the final 30). The inciting incident landing somewhere around page 10 to 15, Plot Point One somewhere around page 25 to 30, and Plot Point Two somewhere around page 55 to 60.
The paradigm provided some useful guideposts for screenwriters trying to fill the empty page, but act two, at 60 pages—roughly half the running length of the film—remained a wasteland of “complications ensue,” with really no more help than that at separating the end from the beginning. Hence the perennial problem of the saggy middle (or “no second act” as they say in The Player). Field tried to address this by expanding the paradigm to include a midpoint and two pinches, but these were arbitrary arithmetic divisions with with no real structural connection to the story.
Then along came Christopher Vogler with his memo introducing the Hero’s Journey to Hollywood—“A Practical Guide to Joseph Campbell’s The Hero With a Thousand Faces.” Yes, George Lucas got there first, leaning heavily on Campbell’s paradigm for Star Wars, but he was perceived as a one-off. Vogler made a convincing case that nearly every successful film owed a debt to Campbell. More importantly for Hollywood, he made the case for using the Hero’s Journey proactively as a blueprint for success. Vogler later expanded his memo to book length—The Writer’s Journey. 
The Best Book on Screenwriting Ever Published
Now the stage is set for Blake Snyder’s awkwardly titled Save the Cat!: The Last Book on Screenwriting You’ll Ever Need.  Snyder stripped away the mythology and archetypes of Campbell and boiled the Hero’s Journey down to thirteen essential beats, then added two of his own, exclusive to film—the opening image and the final image—for a total of fifteen beats. Snyder filled in the standard three-act structure with additional story beats (as opposed to mileposts) that assist the writer struggling in the wasteland, but also ensure the story has a familiar and pleasing shape.
It’s tempting (but foolish) to dismiss this as writing to a “formula.” People made the same argument about Syd Field’s “paradigm.” There were vigorous debates in the industry about the wisdom of writing to “the paradigm.” (I have many, many books of interviews with working screenwriters covering exactly this territory.) But Field’s paradigm was just a notch more advanced than beginning-middle-end. If you think you can write a story without adhering to that convention, you’re probably deluding yourself.
Save the Cat is couched in Hollywood lingo and makes some grandiose claims that can be off-putting, but it’s also immensely practical. Do this, not that. Structure your story like this, not that. Hit these beats, and you’re done. It’s also short, accessible and fun. It may not be the last book on screenwriting you’ll ever need, but it should be the first one you reach for.
The Best Book on Screenwriting Never Published
Wordplay by Terry Rossio. Rossio is one half of the duo responsible for the Pirates of the Caribbean franchise, The Mask of Zorro, The Lone Ranger, Aladdin, Shrek, The Road to El Dorado and the upcoming Godzilla vs. Kong. Yeah, there are some movies on that list that are not to my taste, as I’m sure there are for you, but there are several that I really like. More important, there are at least a half-dozen that a major studio was willing to wager $100,000,000+ budgets on. Let that sink in for a moment. When an editor buys your novel, they’re committing to, maybe, a five-figure advance, a few months of an editor’s time and a slot in their publishing schedule. When a studio greenlights the screenplay for a tentpole picture, they’re committing to somewhere between 1000 times and 10,000 times that amount in production costs, a substantial fraction more for marketing, plus, in some cases, literally centuries in man-years of effort. That’s a strong vote of confidence that you’ve delivered a story that works.
The “book” is actually a series of columns on a website (55 out of a planned 100 so far, but the site hasn’t been update since 2016, and the previous post was in 2014, so don’t wait for him to finish before you start reading). The content swings varies between craft (The Off-Screen Movie) and career advice (Time Risk). They’re long, considerably longer than a typical blog post, but well worth reading. 
The Writer’s Journey by Christopher Vogler. If you’re sold on the Hero’s Journey, but you want a little more meat than Snyder provides.
Adventures in the Screen Trade by William Goldman. Goldman, if you didn’t know, wrote The Princess Bride (the novel and the film), Butch Cassidy and the Sundance Kid, Maverick and another dozen films you’ve probably heard of (and two dozen more you may not have). He knows a thing or two about story. This is more a memoir than a craft book, but it’s more industry focused than personal, and the book on the behind the scenes of screenwriting. If you’ve ever heard anyone say, “No one knows anything” in reference to Hollywood, they’re quoting Goldman from this book.
Save the Cat! Strikes Back by Blake Snyder. The third book of the Save the Cat! trilogy. Includes a fantastic section on loglines and pitches, with templates for both. Most writers leave this till they’re finished writing, only to discover they’ve written something they can’t pitch (or something that doesn’t conform to the pitch they’ve come up with). Snyder’s suggestion is to perfect the pitch first, before committing to the grueling work of writing the screenplay. 
 Field’s books still rank high on searches and are often recommended, but don’t waste your time unless you’re in the mood for a history lesson. They’re not bad, just outdated. The world has moved on.
 I had the great fortune to meet Christopher Vogler at the Maui Writer’s Conference. I pitched him something I was working on (I can’t remember what). His response was some gracious paraphrase of “Have you read my book? Maybe start there.” Which was sad, because I had read it, just not absorbed it.
 Awkward for the exclamation point slammed up against the colon, but also because Snyder wrote two sequels, putting the lie to the subtitle “the last book on screenwriting you’ll ever need.” Awwwwk-ward.
 I stumbled on Wordplay around the time Elliot and Rossio were writing Mask of Zorro, or perhaps just after it came out, so it was a thrill to be reading along when they rose to prominence in the wake of the success of the first Pirates of the Caribbean.
 That exclamation point doesn’t get any less awkward, does it? The second book in the series, Save the Cat! Goes to the Movies is, in my opinion, entirely skippable, being just the application of the beat sheet to a number of familiar movies.