Reddit reviews Three Uses of the Knife: On the Nature and Purpose of Drama

The Power of the Playwright's Vision by Gordon Farrell: I have the privilege of being able to learn from Gordon Farrell directly this year (he is, among other things, a professor at NYU's Dept. of Dramatic Writing) and I cannot endorse his genius enough. His understanding of the techniques and mechanisms that playwrights use in their craft is mind-boggling, and they're catalogued in this book in a surprisingly digestible way. The best thing about this book is that it isn't prescriptive; Farrell doesn't tell you "this is how you write a naturalist play" or "this is where you would always put a reversal." Rather, he familiarizes you with all the tools you can use to write an effective script and how to combine them. To me, this is as good as it gets.

Three Uses of the Knife by David Mamet: This is a slightly more controversial (and much shorter) text, in which famed playwright David Mamet sits down and tells you what it's all about, man. It's rather rambly, and some people say it's contradictory, but I absolutely love the way he breaks down dramatic technique in informal ways. It's tangential and a bit of a mess at times, but you get a sense of why Mamet's plays are so damn good. Farrell's guide you understand rationally, this one you just experience.

Those are my two favorites, and I highly recommend you read both.

EDIT: Forgot about a very important piece of information that you probably already know but that I'll toss in regardless. Reading books on playwriting is a good way to get better, but the best way to get better is to write more plays. There is no better teacher than experience (read: catastrophic failure). Yes, reading books is a great way to understand the techniques and basic structure you can use, but don't cling to the things you read about. Martin McDonagh--arguably today's most successful playwright--came to prominence by writing plays that actually disrespected theatrical convention through his elaborate staging and action sequences (smashing skulls with hammers, shooting cats, shooting an oven with a shotgun). He didn't study classical playwriting techniques, he watched a lot of movies and read Jorge Luis Borges and brought that to the stage.

Basically what I'm saying is: finding your voice is worth a great deal more than learning the "right" way to write.