Its brevity aside, the most striking feature of Jeffrey Sweet's The Dramatist's Toolkit is that it's not written like most playwriting texts. Instead of ponderously essaying Aristotle (though if you want a good analysis of Poetics, I'd recommend Hatcher's Art & Craft of Playwriting), Sweet cuts to the chase of negotiations and subtext. His is the only book on playwriting that I've ever seen to really get to the heart of creating scenes (and the better part of the book focuses on just this).
Because of this emphasis, this is not going to be the book you should refer to for structure; again, Hatcher is strong there, as is Gordon Farrell's Power of the Playwright's Vision (the only text I've seen to give numerous structural breakdowns). However, Sweet's advice - taken well, and with the usual pinch of salt - is very liberating to the writer who's been struggling against formulaic playwriting texts.
Sweet's book is worth its cover price based solely on its discussions of negotiations, which really are eye-openers as to how subtext really works. His section on exposition, with a discussion on high and low contexts, is likewise strong. The book is great as a reference while doing your actual writing, just to remind you of things you can really make work.
Now, Sweet's a bit brief (but powerful) in this book, but makes up for it in the sequel, Solving Your Script. The latter is a reinforcement of the ideas from The Dramatist's Toolkit, with enlightening, well annotated examples. Together, they make up a powerful combination addressing what is so rarely touched on in playwriting books: how to actually write powerful, subtle scenes. Combine them with a couple of good structural books like Hatcher or Farrell, and you've got a much better basis than most of the formulaic texts.